Single mode laser with large optical mode size

ABSTRACT

A laser including a grating configured to reduce lasing threshold for a selected vertically confined mode as compared to other vertically confined modes.

CLAIM OF PRIORITY

This application is a continuation in part of the U.S. Appl. No. 17/898305 filed on Aug. 29, 2022 which is a continuation of the U.S. Appl. No. 16/823823 filed on Mar. 19, 2020 that is Issued as the U.S. Pat. No. 11431149, on Aug. 30, 2022 and claims the benefit of priority to U.S. Provisional Appl. No. 62/822,677 filed Mar. 22, 2019. Each of the applications referenced in this paragraph is hereby incorporated herein by reference in its entirety.

BACKGROUND Field

This application relates generally to single-mode lasers.

Description of the Related Art

Lasers are widely used in telecommunications, sensing, and test and measurement applications. Many high-power lasers are not single-mode while many single-mode lasers do not provide high optical powers.

SUMMARY

High-power single-mode lasers that are capable of providing high optical power and single mode operation can be useful for many applications. Example embodiments described herein have several features, no single one of which is indispensable or solely responsible for their desirable attributes. Without limiting the scope of the claims, some of the advantageous features will now be summarized.

Certain embodiments provide a laser comprising a waveguide configured to support a vertically confined fundamental optical mode and at least one vertically confined higher order optical mode. The laser further comprises an active region at a first position with respect to the waveguide. The laser further comprises a grating (also referred to as grating layer) at a second position with respect to the waveguide. The first position of the active region and the second position of the grating are configured to reduce a first lasing threshold for the fundamental optical mode and to increase a second lasing threshold for the at least one higher order optical mode.

Certain embodiments provide a method for designing a laser comprising a waveguide, an active region, and a grating. The method comprises providing a position of the active region and a position of the grating. The method further comprises calculating at least a vertically confined first optical mode and at least one vertically confined second optical mode supported by the waveguide for the position of the active region and the position of the grating. The method further comprises adjusting the positions of the active region and the grating such that a first product of an overlap of the first optical mode with the grating and an overlap of the first optical mode with the active region is greater than a second product of an overlap of the at least one second optical mode with the grating and an overlap of the at least one second optical mode with the active region. The method further comprises re-calculating at least the first optical mode and the at least one second optical mode and determining perturbations of at least the first optical mode and the at least one second optical mode resulting from the adjusted positions of the active region and the grating. The method further comprises calculating a difference between the first product and the second product. The method further comprises adjusting, if the difference is less than a threshold value, the positions of the active region and the grating such that the first product is larger than the second product.

Certain embodiments provide a laser comprising a waveguide configured to support a vertical fundamental optical mode and at least one vertically confined higher order optical mode, an active region at a first position with respect to the waveguide, a grating at a second position with respect to the waveguide. At least one of the first position or the second position overlaps with a null of at least one higher order mode.

Certain embodiments provide a laser comprising a waveguide layer configured to support a fundamental vertical optical mode and at least a first higher order vertical optical mode, a first active region at a first position with respect to the waveguide layer, a second active region at a second position with respect to the waveguide layer. The laser further comprises a tunnel junction between the first and the second positions, and a grating at a third position with respect to the waveguide layer. The first and the second positions of the first and the second active regions, and the third position of the grating configured to make a first lasing threshold for the fundamental vertical optical mode smaller than a second lasing threshold for the first higher order vertical optical mode.

Certain embodiments provide a method for designing a laser comprising a waveguide layer, at least two active regions, and a grating. The method comprises providing a first position of a first active region, a second position of a second active region, a third position of the grating; calculating at least a vertically confined first vertical optical mode and at least one vertically confined second vertical optical mode supported by the waveguide layer for the first and the second positions of the first and second active regions and the third position of the grating. The method further comprises adjusting the positions of the active regions and the grating such that a first product of an overlap of the first vertical optical mode with the grating and an overlap of the first vertical optical mode with the first and the second active regions is greater than a second product of an overlap of the at least one second vertical optical mode with the grating and an overlap of the at least one second vertical optical mode with the first and the second active regions. The method further comprises re-calculating at least the first vertical optical mode and the at least one second vertical optical mode and determining perturbations of at least the first vertical optical mode and the at least one second vertical optical mode resulting from the adjusted positions of the first and the second active regions and the grating; calculating a difference between the first product and the second product adjusting, if the difference is less than a threshold value, the positions of the first and the second active regions and the grating such that the first product is larger than the second product.

Certain embodiments provide a laser comprising a waveguide layer configured to support a first vertical optical mode and at least a second vertically confined vertical optical mode, a first active region at a first position with respect to the waveguide layer, a second active region at a second position with respect to the waveguide layer, a tunnel junction below the first position and above the second position and a grating at a third position with respect to the waveguide layer. A first product of an overlap of the first vertical optical mode with the grating and an overlap of the first vertical optical mode with the first and the second active regions is greater than a second product of an overlap of the second vertical optical mode with the grating and an overlap of the second vertical optical mode with the first and the second active regions.

Certain embodiments provide a laser comprising a waveguide layer configured to support a first vertical optical mode and at least a second vertically confined vertical optical mode, a first active region at a first position with respect to the waveguide layer, a second active region at a second position with respect to the waveguide layer, a third active region at a third position with respect to the waveguide layer, a first tunnel junction below the first position and above the second position, a second tunnel junction below the second position and above the third position, and a grating at a fourth position with respect to the waveguide layer. A first product of an overlap of the first vertical optical mode with the grating and an overlap of the first vertical optical mode with the first, second, and the third active regions is greater than a second product of an overlap of the second vertical optical mode with the grating and an overlap of the second vertical optical mode with the first, second, and the third active regions.

Certain embodiments provide a laser comprising a waveguide layer configured to supports at least a fundamental vertical optical mode, a ridge layer above the waveguide layer, a high index layer between the waveguide layer and the ridge layer, the high index layer having a refractive index larger than a refractive index of the ridge layer and a refractive index of the waveguide layer, and at least two active regions within the waveguide layer, the active regions configured to provide optical gain the fundamental vertical optical mode.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following description of the various embodiments, reference is made to the accompanying drawings which form a part hereof, and which show, by way of illustration, various embodiments described herein. Other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.

FIG. 1 schematically illustrates an example slab-coupled optical waveguide laser (SCOWL) architecture.

FIG. 2 schematically illustrates an example super-large optical cavity (SLOC) laser architecture.

FIG. 3 schematically illustrates an example laser with multilayered waveguide in accordance with certain embodiments described herein.

FIG. 4 schematically illustrates an example laser comprising a grating layer in accordance with certain embodiments described herein.

FIG. 5 schematically illustrates a different view of the example laser of FIG. 4 .

FIG. 6 schematically illustrates a cross-sectional view of an example laser having a layered waveguide structure including a grating and an active region in accordance with certain embodiments described herein.

FIG. 7 schematically illustrates a cross-sectional view of an example laser having a layered waveguide structure including a grating configured to limit the number of modes supported in accordance with certain embodiments described herein.

FIG. 8 schematically illustrates a cross-sectional view of an example laser with a large optical cavity that is configured to support multiple modes in accordance with certain embodiments described herein.

FIG. 9 schematically illustrates a cross-sectional view of an example laser with a large optical cavity that is configured to support a single mode in accordance with certain embodiments described herein.

FIG. 10 schematically illustrates a cross-sectional view of another example laser with a large optical cavity that is configured to support a single mode in accordance with certain embodiments described herein.

FIG. 11 schematically illustrates a cross-sectional view of an example laser with a buried waveguide architecture and a large optical cavity that is configured to support a single mode in accordance with certain embodiments described herein.

FIG. 12 schematically illustrates a flow chart illustrating an example iterative method for designing a laser with a large optical cavity configured to support a single mode in accordance with certain embodiments described herein.

FIG. 13 schematically illustrates a cross-sectional view of an example laser comprising a gain coupled grating configured to provide enhanced gain and feedback to the fundamental mode over other higher order modes in accordance with certain embodiments described herein.

FIG. 14 schematically illustrates an example laser comprising a ridge layer above the active region in accordance with certain embodiments described herein.

FIG. 15 schematically illustrates an example laser architecture having two active regions and a thick waveguide layer structure that supports more than one vertical optical mode.

FIG. 16 schematically illustrates an example laser architecture having two active regions and a buried waveguide layer that supports more than one vertical optical mode.

These and other features will now be described with reference to the drawings summarized above. The drawings and the associated descriptions are provided to illustrate embodiments and not to limit the scope of the disclosure or claims. Throughout the drawings, reference numbers may be reused to indicate correspondence between referenced elements.

DETAILED DESCRIPTION

Although certain example embodiments are disclosed below, inventive subject matter extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses and to modifications and equivalents thereof. Thus, the scope of the claims appended hereto is not limited by any of the particular embodiments described below. For example, in any method or process disclosed herein, the acts or operations of the method or process can be performed in any suitable sequence and are not necessarily limited to any particular disclosed sequence. Various operations may be described as multiple discrete operations in turn, in a manner that may be helpful in understanding certain embodiments; however, the order of description should not be construed to imply that these operations are order dependent. Additionally, the structures, systems, and/or devices described herein can be embodied using a variety of techniques including techniques that may not be described herein but are known to a person having ordinary skill in the art. For purposes of comparing various embodiments, certain aspects and advantages of these embodiments are described. Not necessarily all such aspects or advantages are achieved by any particular embodiment. Thus, for example, various embodiments can be carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other aspects or advantages as may also be taught or suggested herein. It will be understood that when an element or component is referred to herein as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements can be present therebetween. For clarity of description, “reflector” or “mirror” can be used interchangeably to refer to an optical element and/or a surface having a reflectivity greater than or equal to about 0.01% and less than or equal to 100%. For example, an optical element and/or a surface having a reflectivity greater than or equal to about 5% and less than or equal to 99%, greater than or equal to about 10% and less than or equal to 90%, greater than or equal to about 15% and less than or equal to 80%, greater than or equal to about 20% and less than or equal to 70%, greater than or equal to about 30% and less than or equal to 60%, or any value in any range/sub-range defined by these values can be considered as a reflector or mirror.

Semiconductor lasers are widely used in many applications ranging from telecommunications to sensing, medical applications, and optical pumping (e.g., pumping other laser mediums or amplifier mediums). Semiconductor lasers can comprise a substrate including an optically active layer thereon. In some implementations, the optically active layer can be configured as an epitaxial layer grown over the substrate using semiconductor growth technology. For many applications, it can be advantageous for these semiconductor lasers to output a single longitudinal mode. One method for achieving single longitudinal mode lasing in III-V semiconductor lasers is by incorporation of a Bragg grating, to create either a Distributed Feed Back (DFB) Semiconductor Laser or a Distributed Bragg Reflector (DBR) Laser. Many embodiments of DBR or DFB laser devices can be configured as edge emitting laser devices that are configured to output light from an edge of the laser device. The plane including the edge can be generally oriented along a direction normal to the substrate of the DBR or DFB laser device. The light output from certain embodiments of the edge emitting DBR and DFB laser devices can be coupled to fibers or other passive waveguide circuits. However, the optical mode output from various embodiments of DFB or DBR lasers can be small and/or asymmetrical and may not be matched with the size and/or shape of optical fibers or waveguides of other passive waveguide circuits into which the light from the DBR or DFB laser device is to be coupled. Accordingly, certain embodiments described herein advantageously provide edge emitting laser devices that are configured to output a single spatial mode having a large size that is matched with the size and/or shape of optical fibers or waveguides of other passive waveguide circuits into which the light from these laser devices is to be coupled.

Certain embodiments described herein provide edge emitting semiconductor laser devices that comprise mode converters that are configured to convert the mode of the light output from the edge emitting semiconductor laser devices to larger and/or a more symmetric shape that can be easily coupled into a glass optical fiber, a plastic optical fibers, a polymer waveguide, a doped glass waveguide, a silicon waveguide, and/or a silicon nitride waveguide. In certain such embodiments, the mode converters can advantageously provide a low loss optical connection between the laser and other optical components.

Without relying on any particular theory, the near field optical mode size, in many implementations of semiconductor lasers comprising a III-V material can be small and elliptical. In many implementations, the full width of the elliptical mode along the minor axis (e.g., the distance along the minor axis between positions at which the intensity is 1/e² of the maximum intensity of the elliptical mode) can be approximately less than or equal to 1 micron and the full width of the elliptical mode along the major axis (e.g., the distance along the major axis between positions at which the intensity is 1/e² of the maximum intensity of the elliptical mode) can be less than or equal to 5 microns. The minor axis of the elliptical mode can be oriented parallel to the crystal growth direction. This type of elliptical mode profile that is compressed along a direction parallel to the crystal growth direction may not be compatible with the size and shape of many implementations of optical fibers and/or doped glass waveguides. For example, many implementations of optical fibers and/or doped glass waveguides can comprise a circular core having a diameter of about 9 microns. Many implementations of SiN_(x) waveguides can be configured to have an elliptical cross-sectional shape such that an optical mode having an elliptical shape mode can be efficiently in-coupled into the waveguide. However, even in such implementations, the small size of the optical mode can make the optical alignment process difficult as even a small misalignment can increase optical losses.

One approach to increase the size of the optical mode includes providing a symmetric buried heterostructure waveguide with a tapered section which allows a large optical mode to be supported by a small, symmetric buried waveguide. The size of the optical mode is adiabatically enlarged over the length of the taper. However, some optical loss can be incurred as the optical mode propagates through the length of the taper over which the optical mode is adiabatically enlarged. This approach is advantageous to increase the size of optical modes that are symmetric but have a size less than or equal to about one micron. Other approaches to increase the size of an optical mode can include waveguide couplers in which the optical mode from the laser is coupled vertically or laterally to an adjacent passive waveguide that can support an optical mode with a large optical size. However, these approaches can introduce absorption and scattering optical losses as the optical mode propagates through the length of the passive waveguide over which the mode transfer occurs.

Another approach to increase the size of the optical mode output from a semiconductor laser includes using large waveguide cores. However, this approach may not be practical for single mode operation because most conventional laser designs on semiconductor materials (e.g., materials from the III-V group) become multimode when the waveguide thickness is increased such that the core of the waveguide is enlarged. A first implementation of a semiconductor laser capable of outputting a large and symmetric optical mode is a slab-coupled optical waveguide laser (SCOWL), which uses a large weak confinement slab waveguide beneath the active region. The size of the near-field optical mode in such structures can be 2 to 3 microns in diameter, or even larger. FIG. 1 schematically illustrates an example implementation of a SCOWL comprising a cladding region 103 (e.g., substrate), a waveguide layer 102 (e.g., slab waveguide) over the cladding region 103, an active region 101 over and parallel to the waveguide layer 102, and a ridge 100 over the waveguide layer 102. The active region 101 can comprise quantum wells, a bulk material, quantum dots, quantum lines or quantum dashes that provide optical gain to the laser. The active region 101 can have a higher refractive index than the material of the waveguide layer 102 and the material of the ridge 100. In various implementations, the waveguide layer 102 can comprise a quaternary material (e.g., a combination of indium phosphide (InP) and some other material). In various implementations, the ridge 100 can also comprise quantum wells.

In various implementations, the thickness of the waveguide layer 102 can be between about 0.5 micron and about 20 microns. For example, the thickness of the waveguide layer 102 can be greater than 0.5 micron and less than or equal to 2 microns, greater than or equal to 1.5 microns and less than or equal to 5 microns, greater than or equal to 4 microns and less than or equal to 8 microns, greater than or equal to 7.5 microns and less than or equal to 10 microns, greater than or equal to 9.0 microns and less than or equal to 15 microns. greater than or equal to 12.5 microns and less than or equal to 20 microns, or any value in any range and/or sub-range defined by these values.

In various implementations, the cladding region 103 can comprise semiconductor materials such as, for example, InP, AlGaAs, InGaP, or combinations thereof. The waveguide layer 102 can comprise semiconductor materials such as, for example, InGaAsP, AlInGaAs, AlGaAs or combinations thereof. In implementations in which the cladding region 103 and the waveguide layer 102 comprise AlGaAs, the doping concentration of AlGaAs in the waveguide layer 102 can be different from the doping concentration of AlGaAs in the cladding region 103. The ridge 100 can comprise semiconductor materials such as, for example, InP, AlGaAs having a same doping concentration as the AlGaAs of the cladding region 103, InGaP, or combinations thereof.

In various implementations of the SCOWL, as schematically illustrated by FIG. 1 , an optional passive layer 107 can be disposed over exposed portions of the surface of the semiconductor laser (e.g., excluding surface portions that are configured to provide electrical contact to the various layers and/or regions of the SCOWL). A conducting material (e.g., a metal) 108 can be disposed on surface portions that are configured to provide electrical contact to the various layers and/or regions of the SCOWL. The profile of the fundamental optical mode 104 of the light output from the SCOWL is shown on the left-side of FIG. 1 . The fundamental mode 104 is in the waveguide layer 102 and not localized around the quantum wells of the active region 101. As discussed above, it can be difficult to design the waveguide layer 102 (e.g., having a thickness between 0.5 micron and 20 microns) such that the light output is single mode. Furthermore, it can be difficult to fabricate the thick waveguide layer 102 to have an index of refraction that is less than about 4% of the refractive index of the cladding region 103.

A second implementation of a semiconductor laser capable of outputting a large and symmetric optical mode is a super-large optical cavity (SLOC) laser, an example of which is schematically illustrated in FIG. 2 . Like the SCOWL, a SLOC laser comprises a cladding region 103 (e.g., substrate), a ridge 100, and a waveguide layer 201 between the cladding region 103 and the ridge 100. In the SLOC laser, the active region 101 that provides the optical gain in the laser is positioned within the waveguide layer 201 such that the overlap with the active region 101 of the fundamental mode 104 a supported by the waveguide layer 201 is greater than the overlap with the active region 101 of any of the higher order modes (e.g., higher order than the fundamental mode) supported by the waveguide layer 201 (e.g., second order mode 104 b; third order mode 104 c). The waveguide layer 201 in various implementations of the SLOC laser can be configured similar to the waveguide layer 102 of the SCOWL described above. For example, the waveguide layer 201 can comprise materials similar to the waveguide layer 102 and/or can have a thickness in a range similar to the thickness range of the waveguide layer 102. The waveguide layers in a SLOC laser or a SCOWL can be configured to output light with circular mode profiles. Accordingly, implementations of the SLOC laser and the SCOWL can advantageously provide optical modes having a size and a shape that are compatible to be in-coupled into optical fibers or other waveguide devices with reduced optical losses as compared to lasers comprising tapered mode converters. Moreover, additional epitaxial growth steps are not required as in buried heterostructure spot-size converters.

Certain embodiments described herein utilize laser designs and/or architectures that comprise a waveguide layer having an enlarged thickness (e.g., similar to the SCOWL and SLOC laser architectures described above) and further comprising a grating (e.g., grating layer; grating structure) in the laser cavity to filter the vertically confined modes of the laser down to fewer vertically confined modes (e.g., to a single vertically confined mode). As used herein, the term “vertically confined mode” has its broadest reasonable meaning, including referring to a mode that is confined in a direction parallel to the growth direction of the semiconductor crystal. In some cases, a vertically confined mode can be a vertical optical mode (also referred to as a transverse optical mode) of a waveguide or waveguide layer. A waveguide or waveguide layer may support one or more vertical optical modes each having a different optical field and optical intensity distribution along a vertical direction. The vertical direction (e.g., along y-axis in FIG. 1 ) can be perpendicular to a main surface of a substrate or cladding layer on which the waveguide layer is disposed. The intensity or the magnitude of an optical field associated with a vertical optical mode may have at least one peak value along the vertical direction. The vertical optical modes supported by a waveguide or waveguide layer (also referred to as optical modes of the wave guide or waveguide layer) may include at least a fundamental vertical optical mode and one or more high order vertical optical modes (also referred to as higher order vertical optical modes). An effective refractive index of the fundamental vertical optical mode of a waveguide can be greater than an effective refractive index of a higher order vertical optical mode of the waveguide. For example, the effective refractive index of the fundamental vertical optical mode can be larger than that of a second order vertical optical mode, a third order) vertical optical mode, or higher order vertical optical modes. The effective refractive index of a vertical optical mode of a waveguide can be the speed of light (e.g., in vacuum or air) divided by a velocity of propagation of the vertical optical mode along the waveguide. In some cases, a first vertical optical mode of a waveguide that has an effective refractive index smaller than a second vertical optical mode of the waveguide, is a higher order vertical optical mode compared to the second vertical optical mode. As such, a vertical optical mode that has an effective refractive index greater than all other vertical optical modes of a waveguide, can be the fundamental mode of the waveguide. In some cases, the intensity or the magnitude of an optical field of with the fundamental vertical optical mode may include one peak value along the vertical direction. In some other cases, e.g., when a waveguide layer includes more than one gain layers, the intensity or the magnitude of the optical field of the fundamental vertical optical mode may include more than one peaks along the vertical direction or stay nearly constant or change slowly close to a peak value. In some cases, the intensity or the magnitude of an optical field associated with a higher order vertical optical mode can have at least two peak values along the vertical direction. A higher order vertical optical mode may be herein referred to as “high order vertical optical mode”, “higher order mode”, “high order mode”, or “high order vertical mode”. The fundamental vertical optical mode may be herein referred to as “fundamental vertical mode”, or “fundamental mode”. Various laser structures described herein can be configured to output light having wavelengths between about 200 nanometers and about 8000 nanometers. Certain embodiments described herein comprise a laser that is grown on a substrate comprising GaAs, InP, silicon, or other crystalline materials. Certain embodiments described herein comprise a cladding region that includes materials such as, for example, InP, AlGaAs, GaAs, AlInGaAs, AlInGaP, InGaAsP, InGaP, InGaAs, InAsP. Similarly, certain embodiments described herein comprise a waveguide layer and an active region that comprise any of the materials described above, as well as others, such as GaN, AlGaN. Certain embodiments described herein comprise a grating layer placed so as to suppress lasing of higher order modes and enhance the lasing of a fundamental mode.

FIG. 3 schematically illustrates an example laser in accordance with certain embodiments described herein. The laser comprises a region 103 including a cladding material 103, a layered waveguide structure 105, an active region 101 over the layered waveguide structure 105, and a ridge 100 over the layered waveguide structure 105. The layered waveguide structure 105 comprises a plurality of alternating layers comprising a first material and a second material different from the first material. The plurality of layers comprising the first material can be interleaved with the plurality of layers comprising the second material. In certain embodiments, the first material is the material of the cladding region 103 and the second material has a refractive index higher than the refractive index of the first material. In certain embodiments, the difference between the refractive index of the second material and the refractive index of the first material is less than about 0.4%. In certain embodiments, the thickness of the individual layers comprising the first material and the second material depends on the refractive index of the first and the second material. For example, the thickness of an individual layer comprising the second material can be between about 0.01 micron and about 0.5 micron. By tailoring the ratio of thickness of the layers comprising the first material and the layers comprising the second material, the layered waveguide structure 105 can be configured as a weakly confining waveguide. For example, the ratio of the thickness of the layers comprising the first material and the layers comprising the second material can be between about 1:20 and 20:1 to achieve weak confinement in the waveguide. The layered waveguide structure 105 of certain embodiments advantageously has a refractive index substantially close to the refractive index of the material of the cladding region 103, such as, for example less than 4% of the refractive index of the cladding region 103. The total thickness of the layered waveguide structure 105 can be less than about 20 microns in certain embodiments. For example, the total thickness of the layered waveguide structure 105 can be less than or equal to about 2 microns,, less than or equal to about 5 microns, less than or equal to about 10 microns, and/or greater than 0.5 micron. The total thickness of the layered waveguide structure 105 can have a value in a range/sub-range defined by any of these values. In certain embodiments, the laser of FIG. 3 has an optical confinement low enough to cut-off all higher order modes except the fundamental spatial mode in the X-Y plane parallel to the cross-section shown in FIG. 3 .

In certain embodiments, the laser comprises a grating layer 106 over the active region 101 in the ridge 100, as schematically illustrated by FIG. 3 . The grating layer 106 is configured to reduce the number of lasing longitudinal modes. For example, the grating layer 106 can be configured to reduce the number of lasing longitudinal modes to a single longitudinal mode. In certain embodiments, as schematically illustrated in FIG. 3 , the waveguide has a layered waveguide structure, while in certain other embodiments, the waveguide has a structure similar to that of the waveguide layer 102 schematically illustrated in FIG. 1 .

In certain embodiments, the grating layer 106 is in the active region 101 (e.g., within the active quantum well region), while in certain other embodiments, the grating layer 106 is in the layered waveguide 105, as schematically illustrated in FIG. 4 . The grating layer 106 can be positioned to interact strongly with the fundamental mode (or the first order mode) and to interact weakly (or not interact at all) with the second order mode. For example, the grating layer 106 can be positioned to coincide with the peak of the fundamental mode 104 a and the null of the second order mode 104 b. In certain such embodiments, despite the second order mode 104 b having comparable or better overlap with the active region 101, only the fundamental mode 104 a lases because the feedback from the grating layer 106 for the second order mode 104 b (or the coupling between the grating layer 106 and the second order mode 104 b) is much lower as compared to the feedback from the grating layer 106 for the fundamental mode 104 a (or the coupling between the grating layer 106 and the fundamental mode 104 a). In certain such embodiments, the second order mode 104 b has a higher lasing threshold as compared to the fundamental mode 104 a. For example, in the example laser of FIG. 4 , when the fundamental (e.g., first order) mode 104 a begins lasing, the carrier threshold is approximately clamped, and additional injection current only serves to enhance the power of the fundamental mode 104 a. In certain embodiments, as schematically illustrated in FIG. 4 , the waveguide has a layered waveguide structure, while in certain other embodiments, the waveguide has a structure similar to that of the waveguide layer 102 schematically illustrated in FIG. 1 .

In certain embodiments, the grating layer 106 comprises a material having higher or lower refractive index as compared to the material of the waveguide 105 or the cladding region 103. In certain embodiments, as schematically illustrated in FIG. 5 , the grating layer 106 can be etched in one or more layers of the layered waveguide structure 105. FIG. 5 schematically illustrates a cross-section of the example laser of FIG. 4 in the Y-Z plane with the Z axis oriented along the left to right direction. The ridge etch region 100 is schematically illustrated in FIG. 5 by diagonal line hatching and includes the active region 101. In certain embodiments, as schematically illustrated by FIG. 5 , a single layer of the layered waveguide 105 is etched to form the grating 106, while in certain other embodiments, multiple layers of the layered waveguide 105 are etched to form the grating 106. This approach of etching the grating 106 into one or more layers of the layered waveguide 105 can advantageously allow a single calibration of the example laser during fabrication of both the grating 106 and the layers of the layered waveguide 105.

Growing a thick slab waveguide 102 comprising a material that is different from the material of the cladding region 103 over the cladding region 103 can be difficult and can cause defects in the thick slab waveguide 102. In certain embodiments, the layered waveguide 105 (e.g., comprising relatively thinner layers of the waveguide material alternating with thin layers of the cladding material) is simpler to fabricate than a slab waveguide 102. For example, the layered waveguide 105 can comprise relatively thin layers of a waveguide material comprising quaternary or ternary layers or other layers interleaved with relatively thin layers of InP that are grown on a cladding region 103 comprising InP. The grating etch can etch or punch through one or more of the non-InP layers, resulting in a very well controlled coupling coefficient where the thickness of the grating 106 can be controlled only by the thickness of the non-InP layers. Another stack of relatively thin layers of InP interleaved with relatively thin layers of the waveguide material can be further grown over the waveguide layer or layers comprising the grating 106.

FIG. 6 schematically illustrates an example laser having a SLOC-like laser structure in accordance with certain embodiments described herein. The example laser comprises a layered waveguide 105 configured to form a weakly confining waveguide and can support two or more guided modes (e.g., by appropriate tailoring of the thickness and refractive index of the various layers of the layered waveguide 105). As in a SLOC laser, the example laser of FIG. 6 comprises an active region 101 comprising one or more quantum wells configured (e.g., positioned within the layered waveguide 105) to have a first overlap with the fundamental optical mode 104 a and a second overlap with a higher order mode (e.g., second order mode 104 b), the second overlap less than the first overlap. The example laser of FIG. 6 further comprises a grating layer 106 near the active region 101 and positioned within the layered waveguide 105. In certain embodiments, the grating layer 106 is configured to provide overlap with the fundamental mode 104 a to a larger extent as compared to the higher order mode (e.g., the second order mode 104 b), thereby achieving both higher gain and higher feedback for the fundamental mode 104 a compared to the higher order mode (e.g., second order mode 104 b) which will result in a much lower threshold gain for the fundamental mode 104 a compared to the higher order mode (e.g., second order mode 104 b). In certain other embodiments, the grating layer 106 is positioned elsewhere in the example laser structure, because the second order mode 104 b can experience virtually no gain and will not lase even if the second order mode 104 b experiences a somewhat higher coupling coefficient than does the fundamental mode 104 a.

In certain embodiments, for lower confinement within the quantum wells and/or placement of the quantum wells closer to one side of the waveguide (e.g., at the top near a p-doped side of the waveguide in a laser grown with n-doping on the bottom), the grating layer 106 is positioned at or near a null or minimum of the second order mode 104 b, as schematically illustrated in FIG. 7 . In certain embodiments, the second order mode 104 b can experience slightly more gain than does the fundamental optical mode 104 a, and the grating layer 106 is configured to interact only with the fundamental mode 104 a such that only the fundamental mode 104 a lases. The example laser in FIG. 7 can have a lower quantum well overlap with the lasing mode and thus can be conducive to higher power distributed feedback (DFB) laser designs. In certain embodiments, hole injection to the quantum well and electron confinement in the quantum well can result in better performance with the quantum well or other gain medium near the p-cladding. In certain such embodiments, the grating layer 106 selectively lowers the threshold of the fundamental mode 104 a, despite the placement of the gain nearer to the most concentration of light in the higher order cavity mode (e.g., the second order mode 104 b). In certain embodiments, the active region 101 and the grating layer 106 are configured (e.g., positioned) to select only the second order mode 104 b. In certain embodiments, the active region 101 and the grating layer 106 are configured (e.g., positioned) to select two modes (e.g., fundamental mode 104 a and the second order mode 104 b).

In certain embodiments, the grating 106 in a SLOC-like laser architecture provides an additional parameter to suppress higher order modes to ensure single mode lasing. For example, the active region 101 (e.g., comprising quantum wells) can be placed such that the fundamental mode 104 a experiences higher gain than do other higher order optical modes (e.g., the second order mode 104 b). Tailoring the placement of the grating layer 106 can provide an additional mode selection method to preferentially select the fundamental mode 104 a (or, similarly, to deselect other higher order modes). FIG. 8 schematically illustrates an example laser comprising both an active region 101 and a grating layer 106 within a large optical cavity that is configured to support multiple modes in accordance with certain embodiments described herein. The example laser of FIG. 8 uses both the placement of the active region 101 and the placement of the grating layer 106 to select only the fundamental mode 104 a. For example, the active region 101 of FIG. 8 is positioned within the thick waveguide 201 such that the active region 101 coincides with a null or a minimum of the second order mode 104 b, and the grating layer 106 is positioned within the thick waveguide 201 such that the grating layer 106 coincides with a null or a minimum of the third order mode 104 c. In certain embodiments, the thickness of the waveguide 201 is between about 0.5 micron to 20 microns (e.g., depending on the wavelength of light that the example laser is designed to emit).

In certain embodiments, the active region 101 and/or the grating layer 106 is configured (e.g., positioned) to suppress lasing of one or more modes (e.g., all but the fundamental mode 104 a or some selected higher order mode). For example, as shown in FIG. 9 , the grating layer 106 can be positioned within the thick waveguide 201 to coincide with a null or a minimum of the second order mode 104 b so as to suppress the second order mode 104 b, and the active region 101 can be positioned within the thick waveguide 201 to coincide with a null or a minimum of the third order mode 104 c so as to suppress the third order mode 104 c. FIG. 10 schematically illustrates another example laser in which the active region 101 and the grating layer 106 are both positioned in regions of the thick waveguide 201 that are equally unfavorable to the second order mode 104 b and the third order mode 104 c, while being favorable to the fundamental mode 104 a (e.g., coincide with or overlap with the peak of the fundamental mode 104 a). In certain embodiments, the thick waveguide 201 can comprise a slab of a material such as, for example, InGaAsP or AlInAs. In certain embodiments, the waveguide 201 can be a layered waveguide comprising alternate layers of a first material and a second material, as discussed above in connection with FIGS. 3, 4 and 6 .

The method of designing and/or fabricating a laser by positioning the active region 101 and/or the grating layer 106 to selectively reduce the number of spatial modes supported by the waveguide is not limited to ridge waveguide architectures (e.g., schematically illustrated in FIGS. 3-10 ), but are also applicable to a wide variety of waveguide designs and architectures, including but not limited to a buried waveguide architecture. FIG. 11 schematically illustrates an example laser comprising a buried waveguide 201 that is confined vertically (e.g., in a direction along the growth direction) between the cladding region 103 (e.g., a lower cladding region) and another cladding region 1100 (e.g., an upper cladding region) and confined laterally by regions 112 comprising a material having a refractive index substantially equal to that of the material of the cladding region 103. In certain embodiments, the cladding region 1100 comprises the same material as does the cladding region 103 and/or the cladding region 1100 comprises a material having a refractive index substantially equal to that of the material of the cladding region 103. In certain embodiments, the cladding region 1100 and the regions 112 comprise the same material as does the cladding region 103. In certain embodiments, the material of the regions 112 can also be electrically blocking (e.g., similar to existing buried heterostructure waveguide architectures). The thick waveguide 201 can comprise a bulk material, as discussed above, or can comprise a layered structure (e.g., similar to the layered waveguide 105 discussed above). In certain embodiments, as schematically illustrated in FIG. 11 , the grating layer 106 and the active region 101 are configured (e.g., positioned) to inhibit or prevent lasing of the vertically confined second order mode 104 b and the vertically confined third order mode 104 c (e.g., similar to the example laser schematically illustrated in FIG. 9 ). In certain embodiments not having a ridge (e.g., broad area structures), the grating layer 106 and the active region 101 are configured such that higher order modes in the vertical direction are suppressed.

As described herein, designing and/or fabricating the various example lasers comprises the placement of the grating layer 106 and the active region 101. In certain embodiments in which only two vertical modes that are perpendicular to the direction of the material growth are present, the grating layer 106, the active region 101, or both can be positioned at or near a center of the waveguide so as to coincide with a null or a minimum of the second order mode 104 b. In certain other embodiments in which three vertical modes are present, the grating layer 106 and/or the active region 101 can be offset from the center of the waveguide so as to coincide with a null or a minimum of two higher order modes (e.g., second order mode 104 b; third order mode 104 c). During the design phase, the positions of the grating layer 106 and the active region 101 can be calculated using a mathematical model to simulate the modes which are supported by the waveguide, and the positions of the active region 101 and the grating layer 106 can be iteratively changed relative to the peaks and nulls of the fundamental mode 104 a and the higher order modes. Without relying on any particular theory, the position of the active region 101 can perturb the mode profile significantly as a result of its thickness and relatively high index of refraction. The grating layer 106, however, provides a small perturbation to the mode profile, and can be moved within the waveguide without significantly altering the nature of the supported modes. In certain embodiments, the iterative process can be advantageous to improve or optimize the position of the active region 101. In certain embodiments, the placement of the grating layer 106 can be calculated initially and does not change much during the iterative process.

FIG. 12 is a flow chart that illustrates an example iterative method for designing a laser in accordance with certain embodiments described herein. The method comprises determining the position of the active region 101 and the grating layer 106 to selectively reduce the threshold for lasing of a single vertically-confined mode (e.g., the fundamental mode 104 a) while suppressing lasing of other vertically-confined modes (e.g., second order mode 104 b; third order mode 104 c). For example, the position of the active region 101 and the grating layer 106 can be selected to reduce the lasing threshold for the fundamental mode 104 a. In certain embodiments, the iterative method of FIG. 12 increases or maximizes the combined effect of the coupling coefficient of the grating layer 106 (also referred to as grating feedback) proportional to Γ_(grat) (e.g., the overlap of an optical mode with the grating layer 106) and gain from the active region 101 proportional to Γ_(qw) (e.g., the overlap of the optical mode with the active region 101) for the fundamental mode 104 a. In certain embodiments, Γ_(grat) and Γ_(qw) are calculated for each mode by measuring the portion of the mode that is within the grating layer 106 or the active region 101 of the waveguide. If Γ_(qw) and/or Γ_(grat) can be kept very low for higher order modes as compared to their values for the fundamental mode 104 a, the lasing threshold for the fundamental mode 104 a can be much lower as compared to the lasing threshold for other modes and only the single fundamental mode 104 a is supported. In certain embodiments, the product Γ_(qw) x Γ_(grat) for the fundamental mode 104 a can be in a range of about 1-20 dB (e.g., 1 dB, 3 dB, 10 dB, 15 dB, 20 dB or any value in a range/sub-range defined values between 1 dB and 20 dB) greater than the product Γ_(qw) x Γ_(grat) for other higher order modes present in the waveguide to selectively reduce the lasing threshold for the fundamental mode 104 a.

In an operational block 1101, the example method of FIG. 12 comprises calculating the different modes that are supported by an optical waveguide for an initial position of the active region 101 and the grating layer 106. In an operational block 1103, the example method further comprises moving the positions of the active region 101 (e.g., comprising one or more quantum wells, a bulk material, quantum dots, quantum lines or quantum dashes that provide optical gain to the laser) and the grating layer 106 within the waveguide such that the product Γ_(qw) x Γ_(grat) for a desired mode (e.g., fundamental mode 104 a) is greater than the product Γ_(qw) x Γ_(grat) for one, two, three, four or all other modes supported by the waveguide. In an operational block, 1105, the example method further comprises calculating the different modes supported by the waveguide again to determine the perturbation of the different modes resulting from a change in the position of the active region 101 and the grating layer 106. In an operational block 1107, the example method further comprises calculating a difference between the product Γ_(qw) x Γ_(grat) for a desired mode (e.g., fundamental mode 104 a) determined in block 1105 and the product Γ_(qw) x Γ_(grat) for one, two, three, four or all other modes supported by the waveguide determined in block 1105. In an operational block 1109, the example method further comprises adjusting, if the difference calculated in block 1107 is less than a threshold value (e.g., 1 dB, 3 dB, 10 dB, 15 dB, 20 dB, or any value in a range/sub-range defined by any of these values), the positions of the active region 101 and the grating structure 106 such that the product Γ_(qw) x Γ_(grat) for the desired mode (e.g., fundamental mode 104 a) is larger than the product Γ_(qw) x Γ_(grat) for the one, two, three, four or all other modes supported by the waveguide. If the difference calculated in block 1107 is greater than a threshold value (e.g., 1 dB, 3 dB, 10 dB, 15 dB, 20 dB, or any value in a range/sub-range defined by any of these values), then the positions of the active region 101 and the grating structure 106 can be considered to be optimized.

In certain embodiments, as schematically illustrated by FIG. 13 , the laser comprises an active region 101 and a grating layer 106 that are combined together (e.g., which can be referred to as a gain-coupled laser). For example, the active region 101 and the grating layer 106 can be combined together by partially or completely etching away the gain region from certain portions of the active region 101. In certain embodiments, as schematically illustrated in FIG. 13 , the active region 101 and the grating layer 106 are placed together within the thick waveguide 201 at a position that overlaps with the peak of the fundamental mode 104 a and thus selectively reduce the lasing threshold for the fundamental mode 104 a and suppresses other higher order modes.

In certain embodiments, as schematically illustrated by FIG. 14 , a high power single mode laser utilizes a higher index portion of cladding within the ridge 100. This example laser is like a hybrid between the SLOC and SCOWL architectures, with the active region 101 below the ridge 100, and a region 109 comprising a material having a refractive index greater than that of the cladding material of the ridge 100, as schematically illustrated in FIG. 14 . This architecture is different from the architecture of the SCOWL in that the active region 101 is below the ridge 100 and is different from the SLOC laser architecture in that the ridge 100 comprises a high refractive index material as well as cladding material. Certain such embodiments have an advantage over the SCOWL architecture in that the active region 101 is not etched through by the ridge 100, leading to easier processing and better reliability. In certain embodiments, the waveguide 105 of the laser schematically illustrated in FIG. 14 is configured to support only a large single optical mode and to not support any higher order modes. In some such embodiments, the corresponding laser may generate an optical output beam having a single vertical mode profile without the need for including a grating layer within the laser cavity.

In certain embodiments, the waveguide 105 of the laser schematically illustrated in FIG. 14 supports two, three or more modes, and the placement of the grating layer 106 and/or the grating layer 106 is selected (e.g., optimized) to allow lasing of a desired mode. In certain embodiments, the waveguide 105 has a layered waveguide structure, as schematically illustrated in FIG. 14 , while in certain other embodiments, the waveguide 105 has a slab waveguide structure comprising a material having a refractive index less than the refractive index of the region 109. In certain embodiments, the grating layer 106 is above or below the active region 101, or within the ridge 100. In certain embodiments, the grating layer 106 is configured (e.g., positioned) to reduce or minimize overlap with higher order modes while increasing or maximizing overlap with the fundamental mode 104 a, thereby providing for lasing of the fundamental mode 104 a.

In certain embodiments, a laser (e.g., a semiconductor laser) may include two or more spatially separated active regions or layers that provide optical gain to one or more optical modes (e.g., vertical optical modes) confined in a waveguide layer of the semiconductor laser. In some examples, a laser may comprise multiple active regions within a thick waveguide layer that supports multiple vertical optical modes. In some examples, each active region may comprise one or more quantum wells (or quantum well layers) that can provide optical gain to light associated with different vertical optical modes of the waveguide layer, upon being pumped, e.g., by an electric current (also referred to as injection current). The quantum wells may be configured to provide optical gain within an operational wavelength range of the laser. The laser cavity may support one or more vertical optical modes having wavelengths within the operational wavelength range of the laser. In various embodiments, two different active regions of a laser may comprise different numbers of quantum wells and/or provide different magnitudes of optical gain to a vertical optical mode of the laser. In some cases, the optical gain that an active region provides to a vertical optical mode is proportional to an overlap (e.g., an overlap integral) between the vertical optical mode and the active region. In some cases, two active regions of a laser may be separated by a separation region or layer that does not provide optical gain at least within the operational wavelength range of the laser or a within a wavelength range that can be amplified by the active regions. In some cases, the separation region or layer between the two active regions may comprise a carrier transport region that allows the electrons and/or holes to flow from one of the active regions to the other active region. In some cases, the carrier transport region may comprise a tunnel junction.

As described above, a grating layer (also referred to as grating) may provide optical feedback (e.g., in a longitudinal direction parallel to a direction of propagation of light in the laser cavity) to one or more optical vertical modes of the waveguide layer (e.g., to a corresponding light beam having a vertical field distribution according to the corresponding vertical optical modes). A laser having two or more spatially separated active regions may comprise one more features described above with respect to the lasers having a single active region. For example, the active regions and the grating layer may be positioned within the laser waveguide so as to reduce the lasing threshold for a first vertical mode (e.g., a threshold injection current that can cause the first vertical mode to lase) compared to that of a second vertical mode supported by the waveguide layer of the laser. In some examples, the first vertical mode can be the fundamental vertical mode of the waveguide layer and the second vertical mode can be a high order vertical mode of the waveguide layer (e.g., the first, second, or the third order mode).

In various implementations, the grating layer may coincide with a null or a minimum of the intensity profile of a first high order mode, while one or both gain regions coincide with a null or a minimum of the intensity profile for a second high order mode different from the first high order mode. In some cases, the active regions and the grating layer may be positioned in regions of the waveguide layer that are unfavorable to one or more high order modes while being favorable to the fundamental vertical mode, such that a threshold current required for oscillation of the fundamental vertical mode in the laser cavity is less than those of the higher order modes.

In some cases, the one or more active regions may be arranged to allow an electric current to sequentially pass through the active regions (e.g., along a vertical direction extended from top electrode to a substrate of the laser. In some such cases, at least one tunnel junction (e.g., a pn semiconductor tunnel junction) may be included between at least two active regions (or layers) to enable electric current (e.g., charge carriers) to flow from one of the active regions to another active region (e.g., a subsequent active region). The tunnel junction may not provide optical gain at least within an operational wavelength range of the corresponding laser or a wavelength range that can be amplified by the active regions.

Generating and amplifying laser light using multiple active regions may reduce the electric current required for generating a desired optical output mode (e.g., the fundamental mode) having a desired output power. In some cases, the output power of a first laser having multiple active regions can be larger than that of a second laser having one active region but otherwise being identical to the first laser. For example, for pulsed operation, an amount of optical power generated for a given peak current of a pulsed current, may be increased by including plurality of active regions that are serially pumped by a pulsed injection current, across the laser waveguide and the laser cavity. In some embodiments, the electric current may sequentially pass through the active regions via tunnel junctions disposed between the active regions. A tunnel junction may comprise at least one pn junction configured to enable transmission of charge carriers (e.g., electrons or holes) between n-type and p-type regions of the tunnel junction via quantum tunneling, thereby enabling current to pass from n-type semiconductor to p-type semiconductor (or visa versa). In some cases, the plurality of active regions may comprise a plurality of active regions parallel to a substrate on which the laser is grown or fabricated, or a lower cladding on which the waveguide layer is disposed. A tunnel junction may be positioned or disposed between two subsequent (e.g., two consecutive) active regions where a first active region is above the tunnel junction and a second active region is below the tunnel junction.

In some cases, multiple active regions in a laser cavity may provide more gain to a high order vertical mode of the corresponding waveguide layer compared to the fundamental vertical mode of the waveguide layer. As such, the high order mode may have a lower threshold compared to the fundamental vertical mode, if both modes receive the same level of optical feedback. So while multiple gain regions may generate a larger optical output power (compared to a single gain region), the corresponding optical output beam may have a high order vertical intensity profile comprising two or more peaks and at least a low power region (a null) along the vertical direction. Some of the designs and methods described above with respect to placement of a grating layer within the waveguide layer may be used to reduce the lasing threshold for the fundamental vertical mode and thereby maintaining a fundamental vertical optical output intensity profile in a laser cavity having multiple gain regions. As such, some of the laser designs proposed and described below, may generate an optical output having a fundamental vertical intensity profile but with a larger optical power compared to that of a laser having a single gain region (described above), for a given peak current provided to both lasers. In some examples, the optical output may comprise pulsed optical power. Some of the laser designs proposed and described below, may comprise vertically stacked quantum wells and tunnel junctions.

In some cases, the electrical current injected into the one or more gain regions is turned on and off (e.g., periodically) to generate one or more laser pulses. Given the difficulty of generating short electrical pulses having high peak currents, in some cases, several gain regions separated by tunnel junctions may be included in the laser cavity to reduce the magnitude of a current needed to generate a laser pulse having high peak power. In some cases, the gain regions and the tunnel junctions can be monolithically stacked in series.

In some cases, the slope efficiency of a laser comprising multiple gain regions may scale (e.g., linearly) with a number (N) of the gain regions included in the laser cavity (e.g., within the waveguide layer). In some cases, the output optical power of such laser may increase by N-fold compared to the output optical power of a laser having a single active region (e.g., the lasers described above with respect to FIGS. 1-14 ) at the same injection current. In some cases, the penalty for the increased optical output power may include a higher (e.g., N-fold higher) peak voltage required to generate the current across the multiple active layers.

FIG. 15 schematically illustrates a vertical cross-section of an example optical cavity laser 1500 having a thick waveguide layer and two active layers 101 a, 101 b. The laser 1500 may comprise one or more features similar to those of the lasers described above with respect to FIGS. 1-14 . Similar to the SLOC laser shown in FIG. 2 , the laser 1500 comprises: a cladding region 103 (e.g., substrate) including a cladding material, a ridge or region section 100 comprising a ridge material, and a waveguide layer 201 (e.g., a thick waveguide layer) between the cladding region 103 and the ridge 100, comprising a waveguide material. In some cases, a thick waveguide layer may support more than one vertical optical mode. A top electrode 108 is disposed on the ridge 100 and is configured to provide electrical contact to the ridge 100. The top electrode 108 may comprise a conducting material (e.g., a metal). In some cases, the refractive index of the waveguide material is greater than the refractive indices of the cladding material and the ridge material. In some examples, the ridge material is substantially identical to the cladding material. In some examples, the ridge material and the cladding material comprise same elements with different stoichimometric ratios. The laser 1500 includes a first active region (or layer) 101 a and a second active region (or layer) 101 b positioned within the waveguide layer 201. Additionally, the laser 1500 may include a tunneling region 1502 positioned between the first active region 101 a and the second active region 101 b to allow electric current to flow between these active regions. The tunneling region 1502 can include a tunnel junction (e.g., a semiconductor tunnel junction). In some examples, the tunnel junction can be a pn junction (a junction between a p-type and n-type semiconductor materials). The waveguide layer 201 may comprise a first waveguide sub-layer 201 a above the tunneling region 1502 and a second waveguide sub-layer 201 b, below the tunneling region 1502. The first active region 101 a can be within the first waveguide sub-layer 201 a and the second active region 101 b can be within the second waveguide sub-layer 201 b. An optional passive layer 107 can be disposed over exposed portions of the surface of the semiconductor laser (e.g., excluding surface portions that are configured to provide electrical contact to the various layers and/or regions of the laser 1500). In certain embodiments, the thickness of the waveguide layer 201 can be from 0.5 micron to 20 microns (e.g., depending on the wavelength of light that the example laser is designed to emit).

The tunneling region 1502 may comprise a material similar or different from the waveguide material. In some examples the tunneling region 1502 and the waveguide layer may comprise the same compound semiconductor but with different stoichiometric ratios. In some cases, the tunneling region 1502 may comprise a highly doped material having a dopant concentration larger than that of the waveguide material. In some examples, a dopant (e.g., p or n dopant) concentration in the tunneling region 1502 can between 4 to 6 times, 6 to 10 times, 10 to 20 times, 20 to 50 time, or 50 to 100 time larger than that of the waveguide region 201. In some cases, the tunneling region 1502 may comprise two or more layers having different types of dopants and/or doping concentrations, and/or different materials (e.g., semiconductor materials). The first and the second active regions 101 a, 101 b may positioned such that they provide optical gain to at least one optical mode confined in the waveguide layer 201, upon being pumped. In some implementations, at least one active region may be within the ridge 100. In some implementations, at least one active region may coincide with a peak of an intensity profile (vertical profile) associated with a vertical mode of the waveguide layer 201.

In some cases, the active regions 101 a and 101 b may comprise an undoped or lightly doped regions. In some cases, the undoped or lightly doped region may comprise one or more quantum wells configured to provide optical gain upon being pumped by a current flowing through the corresponding gain region. The active regions 101 a and 101 b may comprise the same or different number of quantum wells.

In some implementations, a portion of the first waveguide region 201 a, above the first active layer 101 a, a portion of the second waveguide region 201 b, below the tunneling region 1502 and above the second active layer 101 b, may comprise a first dopant type (p or n). In some implementations, a portion of the first waveguide region 201 a, below the first active layer 101 a and above the tunneling region 1502, and a portion of the second waveguide region 201 b below the second active layer 101 b, an above the cladding material, may comprise a second dopant type (p or n) different from the first dopant type.

The waveguide layer 201 can comprise materials similar to the waveguide layer 102 (in FIG. 1 ) and/or can have a thickness in a range similar to the thickness range of the waveguide layer 102. Implementations of the laser 1500 can advantageously provide output optical beams with intensity profiles having a size and a shape that are compatible to be in-coupled into optical fibers or other waveguide devices with reduced optical losses as compared to lasers comprising tapered mode converters.

In some implementations, the waveguide layer 201 may comprise a layered waveguide similar to the layered waveguide structure 105 described above with respect to FIG. 3 . The layered waveguide may comprise a plurality of alternating layers comprising a first material and a second material different from the first material. The plurality of layers comprising the first material can be interleaved with the plurality of layers comprising the second material. In certain embodiments, the first material is the material of the cladding region 103 and the second material has a refractive index higher than the refractive index of the first material. In certain embodiments, the difference between the refractive index of the second material and the refractive index of the first material is less than about 0.4%. The layered waveguide may weakly confine optical modes and may comprise one or more feature described above with respect to the layered waveguide 105.

In some cases, the first 101 a and the second 101 b the active regions (or gain layers) each may comprise a vertical layered structure comprising 1 to 3, 3 to 5, 5 to 10, 10 to 15, or 15 -20 single quantum wells (SQWs). The tunneling region 1502 is substantially parallel to the gain regions 101 a and 101 b, and may have a thickness along a direction parallel to a growth direction of tunneling region 1502 (along the y-axis). The thickness of the tunneling region can be from 10 to 20 nm, 20 to 30 nm, 30 to 40 nm, 40 to 50 nm or any ranges formed by these values or larger or smaller.

The laser 1500 includes a grating or grating layer 106 that provides optical feedback to one or more vertical optical modes confined by the waveguide layer 201. In some examples, the grating layer 106 may be configured to provide different levels of optical feedback to different vertical optical modes confined in the waveguide layer 201. For example, placing the grating layer 106 in different positions along the vertical direction (e.g., along the y-axis) may generate different levels of optical feedback for different vertical modes. In some cases, the grating layer 106 may be disposed within the waveguide layer 201. In some such cases, the grating layer 106 may be positioned between the first 101 a and the second 101 b gain regions (e.g., within the tunnel junction 1502). In some cases, the grating layer 106 may be aligned with respect to waveguide layer 201 so as to provide more optical feedback to a fundamental vertical mode 1504 a of the waveguide layer 201 compared to higher order vertical optical modes (e.g., the second order vertical mode 1504 b). For example, the grating layer 106 may be disposed at or near a peak intensity or peak magnitude of the optical field associated with the fundamental optical mode. In some cases, the intensity or the magnitude of an optical field associated with the fundamental vertical mode 1504 a (the fundamental mode) and the second order vertical 1504 b can be different from those of the fundamental vertical mode 104 a and the second vertical 104 b described above with respect to FIGS. 1-14 . For example, the intensity or the magnitude of the optical field of the fundamental vertical mode 1504 may include more than one peaks along the vertical direction or stay nearly constant or change slowly close to a peak value.

With continued reference to FIG. 15 , the first and the second gain regions 101 a, 101 b, of the laser 1500 can overlap with the peaks of the electric field magnitude for the second order vertical mode 1504 b but are offset from the peak of electric field magnitude for the fundamental vertical mode 1504 a. As such, the active regions 101 a, 101 b, can provide more optical gain to the second order vertical mode 1504 b compared to the fundamental vertical mode 1504 a. To compensate for the lower optical gain provided to the fundamental mode 1504 a, the grating layer 106 may be positioned at a vertical position (along y-axis) where the peak of the fundamental vertical mode 1504 a and a null of the second order vertical mode 1504 b are located. As such, the grating layer 106 may provide more optical feedback to the fundamental vertical mode 1504 a compared to the second vertical order mode 1504 b. In some examples, the larger optical feedback may compensate for the lower optical gain provided to the fundamental mode 1504 a resulting in a lower lasing threshold (e.g., a lower threshold injection current) for the fundamental mode 1504 a compared to the second order mode 1504 b. As such, when the magnitude of electric current provided to the active regions 101 a, 101 b, is larger than the threshold current for lasing of the fundamental mode 1504 a, the laser 1500 may generate and sustain light within the fundamental optical mode 1504 a resulting in an optical output beam having a cross-sectional modal profile similar to the fundamental mode 1504 a (e.g., a Gaussian or near Gaussian profile, or a profile having a single intensity peak). In some examples, when the fundamental mode 1504 a begins lasing, a carrier threshold can be approximately clamped, and additional injection current only serves to enhance the power of the fundamental mode 1504 a. As such, in some cases, even when the magnitude of the current is larger than the threshold current for lasing of the second order mode 1504 b, the optical gain provided by the gain regions 101 a, and 101 b may be consumed for enhancing the power of the fundamental mode 1504 a. Similarly, when the active regions 101 a, 101 b, and the grating layer 106 are positioned in the waveguide layer 201 to decrease the threshold for lasing of a first vertical mode of the waveguide layer 201 compared to the threshold for lasing of other vertical modes, increasing the current provided to the gain regions above the threshold current of the first vertical mode may only enhance the optical power of the first vertical mode.

The active regions 101 a, 101 b, can be pumped by an injection current flowing from the top electrode 108 toward the cladding 103, passing through the first active region 101 a, the tunneling region 1502, and the second active region 101 b. In some cases, the injection current may be generated by applying a voltage to the top electrode 108. A bottom electrode may be in electrical contact with the cladding 103 and configured to receive the injection current transmitted through the active regions. The bottom electrode may be kept at a lower voltage compared to the top electrode 108. In some cases, the bottom electrode may be connected to an electrical ground.

In some embodiments, the position of the grating 106 and the positions of the first and the second active regions 101 a, 101 b, with respect the waveguide layer 201 may be configured so that their combined effects makes the lasing threshold for the fundamental vertical optical mode 1504 a smaller than a lasing threshold for at least one higher order vertical optical mode (e.g., the second order mode 1504 b). For example, the grating 106, the first active region 101 a and the second active region 101 b, may be positioned such that a lasing threshold for the fundamental vertical optical mode 1504 a is smaller than a lasing threshold for at least one higher order vertical optical mode by a desired amount. In some cases, alignment between the vertical optical modes, the grating layer 106, and the gain region may be designed such that when an injection current larger than a threshold injection current for the fundamental mode is provided to the laser 1500, the higher order mode does not lase while the fundamental mode lases. In some cases, the grating 106, the first active region 101 a and the second active region 101 b, may be positioned such that a lasing threshold for the fundamental vertical optical mode 1504 a is smaller than the lasing thresholds for all higher order vertical optical modes. In some cases, the grating 106, the first active region 101 a and the second active region 101 b, may be positioned such that a lasing threshold for the fundamental vertical optical mode 1504 a is smaller than the lasing thresholds for most higher order vertical optical modes.

In some cases, at least one of the first and the second active regions 101 a, 101 b, may overlap with a peak (e.g., intensity peak) of at least one higher order vertical optical mode (e.g., the second order mode 1504 b). In some examples, the grating layer 106 may overlap with a peak of the fundamental vertical optical mode. In some such examples, at least one of the first and the second active regions 101 a, 101 b, may not overlap with the peak of the fundamental mode. In some cases, the grating 106 may overlap with a null of the at least one higher order optical mode (e.g., the second order mode 1504 b).

A peak of a mode (an optical mode) may include a position where a peak magnitude of the optical field or intensity associated with the mode is located. A null of a mode may include a position where the magnitude of the optical field or intensity associated with the mode is substantially zero.

In various implementations, the active regions 101 a, 101 b, can be positioned symmetrically or asymmetrically along the vertical direction (e.g., along the y-axis) with respect to the tunneling region 1502 or the tunnel junction therein. For example, one of the active regions 101 a, 101 b, can be vertically closer to the tunneling region 1502 or the tunnel junction therein. In some cases, the active regions 101 a, 101 b, can be positioned symmetrically or asymmetrically along the vertical direction (e.g., along the y-axis) with respect to the grating layer 106. For example, one of the active regions 101 a, 101 b, can be vertically closer to the grating layer 106. The grating layer 106 may be disposed inside or outside of the tunneling region 1502. The grating layer 106 can be closer to second waveguide layer 201 b or closer to the first waveguide layer 201 a. The tunneling region 1502 can be closer to second waveguide layer 201 b or closer to the first waveguide layer 201 a. In some implementations, other configurations are possible. In some cases, the positions and the relative alignments of the active regions 101 a, 101 b, the tunneling region 1502, and the grating layer 106 inside or outside of the waveguide layer 201, may be determined for decreasing the lasing threshold for a vertical mode of the waveguide layer 201 compared to other vertical modes of the waveguide layer 201, without any further geometrical constraints.

In some implementations, the designs described above with respect to positioning two or more active regions (e.g., active regions 101 a and 101 b) and the grating layer 106 to selectively reduce a number of vertical modes lasing (oscillating) within a laser waveguide or cavity, is not limited to ridge waveguide architectures (e.g., schematically illustrated in FIG. 15 ), but are also applicable to a wide variety of waveguide designs and architectures, including but not limited to a buried waveguide architecture. FIG. 16 schematically illustrates an example laser 1600 comprising a buried waveguide layer 201 that is confined vertically (e.g., in a direction along the growth direction) between the cladding region 103 (e.g., a lower cladding region) and another cladding region 1100 (e.g., an upper cladding region) and confined laterally by regions 112 comprising a material having a refractive index substantially equal to that of the material of the cladding region 103. In certain embodiments, the cladding region 1100 comprises the same material as does the cladding region 103 and/or the cladding region 1100 comprises a material having a refractive index substantially equal to that of the material of the cladding region 103. In certain embodiments, the cladding region 1100 and the regions 112 comprise the same material as does the cladding region 103. In certain embodiments, the material of the regions 112 can also be electrically blocking (e.g., similar to existing buried heterostructure waveguide architectures). The thick waveguide 201 can comprise a bulk material, as discussed above, or can comprise a layered structure (e.g., similar to the layered waveguide 105 discussed above).

The laser 1600 includes two active regions 101 a, and 101 b that upon being pumped, they can provide optical gain to the optical modes mainly confined within the buried waveguide layer 201. The optical modes are vertically confined by the upper cladding 1100 and the lower cladding 103, and laterally confined by the regions 112. Additionally, the laser 1600 may include a tunneling region 1502 positioned between the first active region 101 a and the second active region 101 b to allow electric current flow between these active regions. The waveguide region 201 may comprise a first waveguide region 201 a above the tunneling region 1502 and a second waveguide region 201 b, below the tunneling region 1502. The first active region 101 a can be within the first waveguide region 201 a and the second active region 101 b can be within the second waveguide region 201 b.

A top electrode 108 is disposed on the upper cladding 1100 and is configured to provide electrical contact to the cladding 1100. The top electrode 108 may comprise a conducting material (e.g., a metal). In some cases, the laser 1600 may include a bottom electrode configured to provide electrical contact to the lower cladding 103. The top electrode 108 may be used to generate an electrical current (an injection current) flowing through the first and the second active regions 101 a and 101 b, to pump these active regions and for providing optical gain to the optical modes of the laser 1600. The electric current may flow from the top electrode, which is kept at a first electric potential, to the bottom electrode, which is kept at second potential different from the first potential. In some cases, the first potential can be higher than the second potential. In some cases, the bottom electrode and or the lower cladding 103 may be connected to ground.

In certain embodiments, as schematically illustrated in FIG. 16 , the grating layer 106 and the active regions 101 a and 101 b are configured (e.g., positioned) to inhibit or prevent lasing of a second (second order) order vertical mode 1504 b, e.g., by increasing its lasing threshold above that of the fundamental vertical mode 1504 a. In certain embodiments, not having a ridge (e.g., broad area structures), the grating layer 106 and the active regions 101 a and 101 b are configured such at least some of the higher order vertical modes (in the vertical direction) are suppressed.

As described above, the first active region 101 a, the second active region 101 b, and the grating layer 106 may be positioned within the waveguide layer 201 in the laser 1500 or laser 1600 such that a threshold (e.g., a threshold injection current) for lasing of a first vertical mode (e.g., the fundamental mode 1504 a) of the waveguide layer 201 is smaller than a threshold for lasing of at least a second vertical mode (e.g., the second order mode 1504 b) different from the first vertical mode. In some examples, the threshold for lasing of the fundamental mode 1504 a can be smaller than that of the second order mode 1504 b and one or more other vertical modes (higher order vertical modes). In some examples, the threshold for lasing of a vertical mode (e.g., the fundamental mode 1504 a) can be larger than all other vertical modes (higher order vertical modes) of the waveguide layer. In some examples, the threshold for lasing of a vertical mode (e.g., the fundamental mode 1504 a) can be larger than most other vertical modes of the waveguide layer. As such, a placement of the first active region 101 a, the second active region 101 b, and the grating layer 106 may result in suppression of the lasing of one or more vertical modes (e.g., the second order mode 1504 b, a third order mode, etc.), while allowing the lasing of the fundamental mode 1504 a. In some cases, for a given placement of the first active region 101 a, the second active region 101 b, and the grating layer 106, providing an injection current having a magnitude larger than the threshold current for the first vertical mode to the laser 1500 or laser 1600, may result in generation of a single-vertical-mode laser beam (e.g., a large area beam or a beam having a large waist size). The single-vertical-mode optical output beam may have a cross-sectional intensity profile associated with the fundamental vertical mode 1504 a (e.g., a Gaussian distribution). In some examples, a portion of the single-vertical-mode output optical beam may include an intensity distribution associated with a higher order vertical mode. In some such examples, a peak or an average magnitude of such intensity distribution may be suppressed by 10 dB, 100 dB, or 150 dB compared to a peak or an average magnitude of intensity distribution associated with the fundamental vertical mode, in the single-vertical-mode output optical beam.

In some cases, in order to generate a single-vertical-mode laser beam, the positions of the first active region 101 a, the second active region 101 b, and the grating layer 106 may be determined via an iterative calculation process. The process may begin with providing a first initial position of a first active region, a second initial position of a second active region, and a third initial position of the grating layer 106, and calculating at least a first vertical optical mode and at least a second vertical optical mode confined by the waveguide layer 201 for the first and the second initial positions of the first and second active regions and the third initial position of the grating layer 106. Next, a first product of an overlap (e.g., an overall integral) of the first vertical optical mode with the grating (Γ_(grt1)) and an overlap of the first vertical optical mode with the first and the second active regions (Γ_(qw11) and Γ_(qw21) respectively), and a second product of an overlap of the at least one second vertical optical mode with the grating (Γ_(grt2)) and an overlap of the at least one second vertical optical mode with the first and the second active regions (Γ_(qw12) and Γ_(qw22) respectively), are calculated. If the first product (Γ_(qw11) + Γ_(qw21) )×Γ_(grt1) is not larger than the second product (Γ_(qw12) + Γ_(qw22)) × Γ_(grt2) by a threshold value, the positions of the active regions 101 a, 101 b, and the grating layer 106 may be adjusted so as to increase the first product and/or decrease the second product. After the first adjustment of the positions the first and the second products are calculated and compared again, if still the first product is not larger than the second product by the threshold value, the positions may be adjusted for a second time. This process may be repeated (iteratively) until the first product becomes larger than the second product by the threshold value. In some cases, the process described above is performed by a computing system comprising a non-transitory memory and at least one processor. The processor may perform the process by executing machine-readable instructions stored in the non-transitory memory. The threshold value may be a value stored in the memory and/or provided by a user via a user interface. In some cases, the processor may determine the threshold value based on a desired or target suppression level of a second vertical mode (e.g., a 2^(nd) or higher order vertical mode) compared to a first vertical mode (e.g., the fundamental mode 1504 a) of the waveguide layer 201. In some cases, in each iteration of the calculation process described above, after adjusting positions and before calculating the first and the second products, the processor may re-calculate the first vertical optical mode and the second vertical optical mode and determine perturbations of the first vertical optical mode and the second vertical optical mode resulting from the adjusted positions of the first and the second active regions and the grating layer. Using this process, the threshold for lasing of a single vertical mode (e.g., the fundamental mode 1504 a) may be selectively reduced and/or the threshold for lasing of other vertical modes (e.g., second order mode 1504 b) may be increased to make the threshold for lasing of the first vertical mode larger than that of the first vertical mode by the threshold amount.

In some examples, the process described with respect to FIG. 12 may be used to design a laser having at least two active regions (e.g., laser 1500 or laser 1600) in accordance with certain embodiments described herein. However, for these lasers, instead of the product Γ_(qw)×Γ_(grat), the product (Γ_(qw1)+ Γ_(qw2))×Γ_(grat) may be used. For example, positions of the active regions 101 a, 101 b and the grating layer 106 can be selected to make the threshold for lasing of the fundamental mode 1504 a less than that of the second mode and/or other higher order modes of the waveguide layer 106. In certain embodiments, the iterative method of FIG. 12 increases or maximizes the combined effect of the coupling coefficient of the grating layer 106 (also referred to as grating feedback) proportional to Γ_(grat) (e.g., the overlap of an optical mode with the grating layer 106), the gain from the first active region 101 a proportional to Γ_(qw1) (e.g., the overlap of the optical mode with the active region 101), and the gain from the second active region Γ_(qw2), for the fundamental mode 1504 a. In certain embodiments, Γ_(grat), Γ_(qw1), and Γ_(qw2) are calculated for an optical mode by estimating the portions of the optical mode that are within the grating layer 106, the active region 101 a, or the active region 101 b, respectively. In some cases, such portions may comprise overlap integrals of the optical mode with the grating layer 106, the active region 101 a, and the active region 101 b, respectively. In some implementations, if at least one of Γ_(qw1), Γ_(qw2), and Γ_(grat) is kept very low for a higher order mode as compared to their values for the fundamental mode 1504 a, the lasing threshold for the fundamental vertical mode 1504 a can be much lower as compared to the lasing threshold for the higher order mode. In some implementations, when at least one of Γ_(qw1)× Γ_(grat), Γ_(qw2)×Γ_(grat), or Γ_(qw1)+Γ_(qw2) is kept very low for higher order modes as compared to its value for the fundamental vertical mode 1504 a, the lasing threshold for the fundamental mode 1504 a can be much lower as compared to the lasing threshold for other modes. In some implementations, if (Γ_(qw1),+ΓF_(qw2))× Γ_(grat) is kept very low for higher order modes as compared to their values for the fundamental vertical mode 1504 a, the lasing threshold for the fundamental vertical mode 1504 a can be much lower as compared to the lasing threshold for other modes. In some cases, when the lasing threshold (e.g., lasing threshold injection current) for the fundamental vertical mode 1504 a is lower as compared to the lasing threshold for other modes, providing an injection current larger than a threshold current to the corresponding laser (e.g., laser 1500 or 1600), may result in generation of a single-vertical-mode laser beam. The threshold current can be an injection current larger than the lasing threshold current for the fundamental mode 1504 a (or a preferred mode).

In certain embodiments, the product (Γ_(qw1)+ Γ_(qw2))×Γ_(grat) for the fundamental mode 1504 a can be in a range of about 1-20 dB (e.g., 1 dB, 3 dB, 10 dB, 15 dB, 20 dB or any value in a range/sub-range defined values between 1 dB and 20 dB) greater than the product (Γ_(qw1)+ Γ_(qw2))×Γ_(grat) for other higher order modes present in the waveguide to selectively reduce the lasing threshold for the fundamental mode 1504 a.

With reference to the process shown in FIG. 12 , at block 1101 several optical modes (e.g., vertical optical mode) that are supported by a waveguide layer 201 for an initial position of the active region 101 a, active region 1002 a, and the grating layer 106, are calculated. In some cases, at block 1101, the waveguide layer 201 supporting multiple vertical modes may be designed. In some cases designing the waveguide layer 201, may comprise determining the thickness of the waveguide layer 201, the thicknesses of the first and second portions 201 a, 201 b of the waveguide layer 201, and the thickness and position of the tunnel region 1502, or any combination of these. In some examples, designing the waveguide layer 201 may also comprise determining a number of quantum wells included in the first and the second gain regions 101 a, 101 b, and/or the widths of these gain regions. At block 1103, the positions of the active regions 101 a, 101 b and the grating layer 106 within the waveguide layer 201 are moved such that the product (Γ_(qw1)+ Γ_(qw2))× Γ_(grat) for a desired mode (e.g., fundamental vertical mode 1504 a) is greater than the product (Γ_(qw1)+ Γ_(qw2))× Γ_(grat) for one, two, three, four or all other modes (e.g., all other vertical modes) supported by the waveguide layer 201. At block 1105, different modes supported by the waveguide layer 201 are recalculated to determine the perturbation of different modes resulting from a change in the position of the active regions 101 a and 101 b and the grating layer 106. At the decision block 1107, an amount (Δ) by which the product (Γ_(qw1)+ Γ_(qw2))× Γ_(grat) for a desired mode (e.g., fundamental mode 1504 a) determined at block 1105 is larger than the product (Γ_(qw1)+ Γ_(qw2))× Γ_(grat) for one, two, three, four or all other modes supported by the waveguide determined at block 1105, is calculated. If Δ is larger than a threshold value (e.g., by 1 dB, 3 dB, 10 dB, 15 dB, 20 dB, or any value in a range/sub-range defined by any of these values) the process is complete and the stopped the positions of the positions of the active regions 101 a, 101 b and the grating layer 106 used to calculate Δ may be as the design parameters. If Δ is smaller than the threshold value the process moves to block 1109, where the positions of the active regions 101 a, 101 b and the grating layer 106 within the waveguide layer 201 are further moved to increase the product (Γ_(qw1)+ Γ_(qw2))× Γ_(grat) for the desired mode and/or decrease the product (Γ_(qw1)+ Γ_(qw2))× Γ_(grat) for one, two, three, four or all other modes (e.g., all other vertical modes). Next process moves back to block 1105 for another iteration where the optical modes are recalculated and Δ is estimated. Other processes may be use and the above processing steps can be added, removed, or reordered.

In some cases, the positions of the grating layer 106 in the waveguide layer 201 of the laser 1500 or 1600, may not overlap with the location of any peak magnitude of the optical field or intensity associated with the fundamental vertical optical mode 1504 a.

In some cases, the positions of the gain regions 101 a and 101 b in the layer 106 in the waveguide layer 201 of the laser 1500 or 1600, may not overlap with the location of any peak magnitude of the optical field or intensity associated with the fundamental vertical optical mode 1504 a.

In some cases, the positions of the grating layer 106 in the waveguide layer 201 of the laser 1500 or 1600, may not overlap with the location of any null of the optical field or intensity associated with the higher order verticals optical modes (e.g., the second vertical optical mode 1504 a).

In some cases, the positions of the gain regions 101 a and 101 b in the layer 106 in the waveguide layer 201 of the laser 1500 or 1600, may not overlap with the location of any peak magnitude of the optical field or intensity associated with the higher order verticals optical modes (e.g., the second vertical optical mode 1504 a).

In some cases, the positions of the grating layer 106 in the waveguide layer 201 of the laser 1500 or 1600, may not overlap with the location of any peak magnitude of the optical field or intensity associated with the fundamental vertical optical mode 1504 a.

In certain such embodiments, the second order mode 1504 b may have comparable or better overlap with one or both the active regions 101 a and 101 b, only the fundamental mode 1504 a lases because the feedback from the grating layer 106 for the second order mode 1504 b (or the coupling between the grating layer 106 and the second order mode 1504 b) is much lower as compared to the feedback from the grating layer 106 for the fundamental mode 1504 a (or the coupling between the grating layer 106 and the fundamental mode 1504 a).

In some cases, the grating layer 106 is positioned within the waveguide 201 layer such that the grating layer 106 coincides with a null or a minimum of the second order mode 1504 b, and at least one of the active regions 101 a and 101 b coincide with a null or a minimum of a third order mode (e.g., mode 104 c shown in FIG. 10 ). In some such cases, a threshold (e.g., a threshold injection current) for lasing the fundamental mode 1504 a can be smaller than that of the second or third order optical modes 1504 a and 104 c.

In various implementations, an optical power of light outputted from the laser 1500 and laser 1600 can be from 10 to 50 W and the placement of the grating 106 and the active regions 101 a and 101 b may result in a side mode suppression ratio of light outputted from the laser 1500 to be from larger than 10 dB, 100 dB, or 150 dB.

In some cases, the laser designs, structures, and design methods described above with respect to laser having two active regions may be used to design lasers having more than two active regions and can generate an optical output beam comprising a single vertical mode or having a vertical intensity profile dominated by a single vertical mode (e.g. the fundamental vertical mode). In some such cases, the active regions may be stacked in a vertical direction (e.g., a direction of along which different layers are disclosed or grown) such that each active region is below a subsequent active region. In some examples, a tunnel junction may be disposed between two consecutive active regions to enable an electric current (e.g., injection current) to serially flow from one active region to another active region. As example laser with multiple active regions may include a waveguide layer configured to support a first vertical optical mode (e.g., the fundamental vertical mode) and at least a second vertical optical mode (e.g., the second or third order optical modes), a first active region at a first position with respect to the waveguide layer, a second active region at a second position with respect to the waveguide layer, a third active region at a third position with respect to the waveguide layer, a first tunneling region below the first position and above the second position, and a second tunneling region below the second position and above the third position. The laser may include a grating at a fourth position with respect to the waveguide layer, which provides optical feedback to one more vertical modes of the waveguide layer. The active regions and the grating layer may be aligned with respect to the a vertical optical field or intensity profile of the first and the second vertical mode such that a first product of an overlap of the first vertical optical mode with the grating and an overlap of the first vertical optical mode with the first, second, and the third active regions is greater than a second product of an overlap of the second vertical optical mode with the grating and an overlap of the second vertical optical mode with the first, second, and the third active regions.

In certain embodiments, the laser 1500 may comprises one or more features described above with respect to the laser in FIG. 14 . For example, the ridge 100 of may include a high index portion having a refractive index large than that of the cladding material (similar to the high index portion 1089 of the laser shown in FIG. 14 ). In the presence of the higher index portion of cladding 100, a vertical size (e.g., along y-axis) of a vertical mode (e.g., the fundamental vertical mode) can be larger than the vertical size of an equivalent mode of the waveguide layer 201 in the absence of the high index region. In some examples, the refractive index of the high index portion can be substantially equal to the refractive index of the waveguide layer 201. In some examples, the refractive index of the high index portion can be larger than the refractive index of the waveguide layer 201. In some cases, the waveguide layer 201 and the high index portion of the ridge may only a single large size optical mode and to not support any higher order modes. The single large size optical may extended to the high index region of the ridge. In some such embodiments, the corresponding laser may generate an optical output beam having a single vertical mode profile without the need for including a grating layer within the laser cavity.

In certain embodiments, the waveguide layer 201 and the high index region of the ridge may support two, three or more modes vertical optical modes, and the placement of the grating layer 106 and the active regions 101 a, 101 b may result in a lower lasing threshold for fundamental vertical mode compared to the higher order modes. The active regions 101 a, 101 b may be disposed positioned within the waveguide layer 201. Various methods and designs described above with respect to making the lasing threshold of the fundamental vertical mode smaller than that than those of the higher order modes may be used for a laser having at least two active regions and a high index region.

Although for various embodiments of lasers discussed herein, the first and the second active regions 101 a, 101 b, the grating layer 106, the tunneling region 1502, and the tunnel junction can be described as being positioned inside the waveguide layer 201, it should be appreciated that one or more of these regions can be outside of the waveguide layer 1502. For example, with reference to FIG. 15 , one or more of the first and the second active regions 101 a, 101 b, the grating layer 106, the tunneling region 1502, and the tunnel junction, can be within the ridge 100 or in a region between the waveguide layer 201 and the ridge 100. As anote example, with reference to FIG. 16 , one or more of the first and the second active regions 101 a, 101 b, the grating layer 106, the tunneling region 1502, and the tunnel junction, can be within the cladding 1100 (upper cladding) or in a region between the waveguide layer 201 and the cladding 1100.

Certain embodiments described herein can be configured to output optical power greater than or equal to about 10 mW (e.g., greater than or equal to about 20 mW, greater than or equal to about 30 mW, greater than or equal to about 50 mW, greater than or equal to about 75 mW, greater than or equal to about 100 mW, greater than or equal to about 150 mW) and/or less than or equal to 50 W (e.g., less than or equal to 25 W, less than or equal to 10 W, less than or equal to 5 W, less than or equal to 1 W), or any optical power in a range/sub-range defined by these values. Certain embodiments described herein are configured to output a single vertically confined mode. Accordingly, the light output from certain embodiments described herein have a large side mode suppression ratio (SMSR). For example, the SMSR of light output from certain embodiments described herein can be between about 10 dB and about 150 dB (e.g., between about 10 dB and about 20 dB, between about 15 dB and about 30 dB, between about 20 dB and about 40 dB, between about 30 dB and about 60 dB, between about 40 dB and about 80 dB, between about 50 dB and about 100 dB, between about 60 dB and about 120 dB, between about 70 dB and about 140 dB, or any value in any range/sub-range defined by these values.).

Although for various embodiments of lasers discussed herein, the active region and/or the grating layer can be described as being positioned at the peak and/or at the null of the vertically confined mode, it should be appreciated that the active region and/or the grating layer can be positioned near or in proximity to the peak and/or near or in proximity to the null of the vertically confined mode to increase/decrease lasing threshold of the vertically confined mode.

In certain embodiments, a computer system is used for some or all of the calculations described herein. For example, the computer system can comprise hardware (e.g., at least one microprocessor) operative to execute software (e.g., code stored on computer-readable non-transitory memory media). It will be appreciated that one or more portions, or all of the code may be remote from the user and, for example, resident on a network resource, such as a LAN server, Internet server, network storage device, etc. In certain embodiments, the computer system comprises a standard personal computer. The computer system can comprise standard communication components (e.g., keyboard, mouse, trackball, touchpad, toggle switches) for receiving user input (e.g., commands and/or data from a human operator), and can comprise standard communication components (e.g., image display screen, alphanumeric meters, printers) for displaying and/or recording output data, and computer-readable non-transitory memory media (e.g., random-access memory (RAM) integrated circuits; hard-disk drives).

EXAMPLE EMBODIMENTS

Various additional example embodiments of the disclosure can be described by the following examples:

Group 1

Example 1. A laser comprising:

-   a waveguide configured to support a vertically confined fundamental     optical mode and at least one vertically confined higher order     optical mode; -   an active region at a first position with respect to the waveguide;     and -   a grating at a second position with respect to the waveguide, the     first position of the active region and the second position of the     grating configured to reduce a first lasing threshold for the     fundamental optical mode and to increase a second lasing threshold     for the at least one higher order optical mode.

Example 2. The laser of Example1, wherein the active region overlaps with a peak of the fundamental optical mode.

Example 3. The laser of Example1, wherein the active region overlaps with a null of the at least one higher order optical mode.

Example 4. The laser of Example1, wherein the grating overlaps with a peak of the fundamental optical mode.

Example 5. The laser of Example1, wherein the grating overlaps with a null of the at least one higher order optical mode.

Example 6. The laser of Example1, wherein an optical power of light outputted from the laser is between about 10 mW and about 50 W.

Example 7. The laser of Example1, wherein a side mode suppression ratio of light outputted from the laser is between about 10 dB and about 150 dB.

Example 8. The laser of Example1, wherein the waveguide comprises a first plurality of layers comprising a first material having a first refractive index and a second plurality of layers comprising a second material having a second refractive index.

Example 9. The laser of Example8, wherein the waveguide is a weakly confined waveguide.

Example 10. The laser of Example1, wherein the active region is over the waveguide.

Example 11. The laser of Example1, wherein the active region within the waveguide.

Example 12. The laser of Example1, wherein the grating structure is within the waveguide.

Example 13. The laser of Example1, wherein the waveguide is between and vertically confined by a first region and a second region, the first region comprising a first material having a first refractive index, the second region comprising a second material having a second refractive index, the waveguide comprising a third material having a third refractive index, the first refractive index less than the third refractive index, and the second refractive index less than the third refractive index.

Example 14. The laser of Example13, wherein the waveguide is laterally confined by at least one region comprising a fourth material having a fourth refractive index.

Example 15. The laser of Example14, wherein the fourth refractive index is substantially equal to the first refractive index and/or the second refractive index.

Example 16. The laser of Example13, wherein the fourth material is electrically blocking or insulating.

Example 17. A method for designing a laser comprising a waveguide, an active region, and a grating, the method comprising:

-   providing a position of the active region and a position of the     grating; -   calculating at least a vertically confined first optical mode and at     least one vertically confined second optical mode supported by the     waveguide for the position of the active region and the position of     the grating; -   adjusting the positions of the active region and the grating such     that a first product of an overlap of the first optical mode with     the grating and an overlap of the first optical mode with the active     region is greater than a second product of an overlap of the at     least one second optical mode with the grating and an overlap of the     at least one second optical mode with the active region; -   re-calculating at least the first optical mode and the at least one     second optical mode and determining perturbations of at least the     first optical mode and the at least one second optical mode     resulting from the adjusted positions of the active region and the     grating; -   calculating a difference between the first product and the second     product; -   adjusting, if the difference is less than a threshold value, the     positions of the active region and the grating such that the first     product is larger than the second product.

Example 18. The method of Example17, wherein the first optical mode is a fundamental optical mode supported by the waveguide and the at least one second optical mode is a second order optical mode and/or a third order optical mode supported by the waveguide.

Example 19. The method of Example17, wherein the position of the active region and the position of the grating are within the waveguide.

Example 20. The method of Example17, wherein the threshold value is 1 dB, 3 dB, 10 dB, 15 dB, 20 dB, or any value in a range/sub-range defined by any of these threshold values.

Group 2

Example 1. A laser comprising:

-   a waveguide layer configured to support a fundamental vertical     optical mode and at least a first higher order vertical optical     mode; -   a first active region at a first position with respect to the     waveguide layer; -   a second active region at a second position with respect to the     waveguide layer; -   a tunnel junction between the first and the second positions; and -   a grating at a third position with respect to the waveguide layer; -   wherein the first and the second positions of the first and the     second active regions, and the third position of the grating     configured to make a first lasing threshold for the fundamental     vertical optical mode smaller than a second lasing threshold for the     first higher order vertical optical mode.

Example 2. The laser of Example 1, wherein at least one of the first and the second active regions overlap with a peak of the first higher order vertical optical mode.

Example 3. The laser of Example 1, wherein the grating overlaps with a peak of the fundamental vertical optical mode.

Example 4. The laser of Example 1, wherein the grating overlaps with a null of the first higher order optical mode.

Example 5. The laser of Example 1, wherein the waveguide layer is configured to support a second higher order optical mode, and the grating overlaps with a null of the first or the second higher order optical mode.

Example 6. The laser of Example 5, wherein the grating overlaps with a peak of the fundamental vertical optical mode.

Example 7. The laser of Example 1, wherein a first product of an overlap of the fundamental vertical optical mode with the grating and an overlap of the fundamental vertical optical mode with the first and the second active regions is greater than a second product of an overlap of the first higher order vertical optical mode with the grating and an overlap of the first higher order vertical optical mode with the first and the second active regions.

Example 8. The laser of Example 5, wherein a first product of an overlap of the fundamental vertical optical mode with the grating and an overlap of the fundamental vertical optical mode with the first and the second active regions is greater than a second product of an overlap of the second higher order vertical optical mode with the grating and an overlap of the second higher order vertical optical mode with the first and the second active regions.

Example 9. The laser of Example 1, wherein the first lasing threshold for the fundamental vertical optical mode comprises a first threshold injection current and the second lasing threshold for the first higher order vertical optical mode comprises a second threshold injection current larger than the first threshold injection current, and wherein an injection current larger than the first threshold current provided to the laser generates an optical output beam, a contribution of the first higher order vertical optical modes in the optical output beam being suppressed with respect to a contribution of the fundamental vertical optical mode by at least 10 dB.

Example 10. The laser of Example 5, wherein the first and the second positions of the first and the second active regions, and the third position of the grating are further configured to make the first lasing threshold for the fundamental vertical optical mode smaller than a third lasing threshold for the second higher order vertical optical mode.

Example 11. The laser of Example 10, wherein the first lasing threshold for the fundamental vertical optical mode comprises a first threshold injection current, the second lasing threshold for the first higher order vertical optical mode comprises a second threshold injection current, the third lasing threshold for the second higher order vertical optical mode comprises a third threshold injection current, wherein the third threshold current is larger than the second threshold injection current and the second threshold current is larger than the first threshold injection current.

Example 12. The laser of Example 11, wherein an injection current larger than the first threshold current is provided to the laser generates an optical output beam, and wherein contributions of the first and the second higher order vertical optical modes in the output optical beam are suppressed with respect to a contribution of the fundamental vertical optical mode by at least 10 dB.

Example 13. The laser of Example 12, wherein the injection current sequentially passes through the first active region, the tunnel junction, and the second active region.

Example 14. The laser of Example 1, wherein an optical power of light outputted from the laser is between about 10 mW and about 50 W.

Example 15. The laser of Example 1, wherein a side mode suppression ratio of light outputted from the laser is between about 10 dB and about 150 dB.

Example 16. The laser of Example 1, wherein the waveguide comprises a first plurality of layers comprising a first material having a first refractive index and a second plurality of layers comprising a second material having a second refractive index.

Example 17. The laser of Example 15, wherein the waveguide layer comprises a weakly confined waveguide.

Example 18. The laser of Example 1, wherein at least one of the first and the second active regions is within the waveguide layer.

Example 19. The laser of Example 1, wherein the grating is within the waveguide layer.

Example 20. The laser of Example 1, wherein the waveguide layer is between and vertically confined by a first region and a second region, the first region comprising a first material having a first refractive index, the second region comprising a second material having a second refractive index, the waveguide layer comprising a third material having a third refractive index, the first refractive index less than the third refractive index, and the second refractive index less than the third refractive index.

Example 21. The laser of Example 20, wherein the waveguide is laterally confined by at least one region comprising a fourth material having a fourth refractive index.

Example 22. The laser of Example 21, wherein the fourth refractive index is substantially equal to the first refractive index and/or the second refractive index.

Example 23. The laser of Example 22, wherein the fourth material is electrically blocking or insulating.

Example 24. The laser of Example 1, wherein the tunnel junction comprises a pn semiconductor junction.

Example 25. The laser of Example 1, wherein the third position is between the first and the second positions.

Example 26. The laser of Example 1, wherein the first position is above the second position.

Example 27. The laser of Example 26, wherein the third position is above the second position and below the first position.

Example 28. The laser of Example 1, wherein the tunnel junction is below the first position and above the second position.

Example 29. The laser of Example 28, wherein the third position overlaps with the tunnel junction.

Example 30. A method for designing a laser comprising a waveguide layer, at least two active regions, and a grating, the method comprising:

-   providing a first position of a first active region, a second     position of a second active region, a third position of the grating; -   calculating at least a vertically confined first vertical optical     mode and at least one vertically confined second vertical optical     mode supported by the waveguide layer for the first and the second     positions of the first and second active regions and the third     position of the grating; -   adjusting the positions of the active regions and the grating such     that a first product of an overlap of the first vertical optical     mode with the grating and an overlap of the first vertical optical     mode with the first and the second active regions is greater than a     second product of an overlap of the at least one second vertical     optical mode with the grating and an overlap of the at least one     second vertical optical mode with the first and the second active     regions; -   re-calculating at least the first vertical optical mode and the at     least one second vertical optical mode and determining perturbations     of at least the first vertical optical mode and the at least one     second vertical optical mode resulting from the adjusted positions     of the first and the second active regions and the grating; -   calculating a difference between the first product and the second     product; -   adjusting, if the difference is less than a threshold value, the     positions of the first and the second active regions and the grating     such that the first product is larger than the second product.

Example 31. The method of Example 30, wherein the first optical mode is a fundamental vertical optical mode supported by the waveguide layer and the at least one second vertical optical mode is a second order vertical optical mode and/or a third order vertical optical mode supported by the waveguide layer.

Example 32. The method of Example 30, wherein the positions of the first and the second active regions and the position of the grating are within the waveguide layer.

Example 33. The method of Example 30, wherein the threshold value is 1 dB, 3 dB, 10 dB, 15 dB, 20 dB, or any value in a range/sub-range defined by any of these threshold values.

Example 34. The method of Example 30, further comprising providing a fourth position of a tunnel junction between the first and the second positions.

Example 35. The method of Example 34, calculating the vertically confined first vertical optical mode and the vertically confined second vertical optical mode comprises, calculating the vertically confined first vertical optical mode and the vertically confined second vertical optical mode for the fourth position of the tunnel junction

Example 36. The method of Example 34, wherein the tunnel junction comprises a pn semiconductor junction.

Example 37. The method of Example 30, wherein the first position is above the second position.

Example 38. The method of Example 37, wherein the third position is above the second position and below the first position.

Example 39. A laser comprising:

-   a waveguide layer configured to support a first vertical optical     mode and at least a second vertically confined vertical optical     mode; -   a first active region at a first position with respect to the     waveguide layer; -   a second active region at a second position with respect to the     waveguide layer; -   a tunnel junction below the first position and above the second     position; and -   a grating at a third position with respect to the waveguide layer; -   wherein a first product of an overlap of the first vertical optical     mode with the grating and an overlap of the first vertical optical     mode with the first and the second active regions is greater than a     second product of an overlap of the second vertical optical mode     with the grating and an overlap of the second vertical optical mode     with the first and the second active regions.

Example 40. The laser of Example 39, wherein the first optical mode is a fundamental vertical optical mode supported by the waveguide layer and the at least one second vertical optical mode is a second order vertical optical mode or a third order vertical optical mode supported by the waveguide layer.

Example 41. A laser comprising:

-   a waveguide layer configured to support a first vertical optical     mode and at least a second vertically confined vertical optical     mode; -   a first active region at a first position with respect to the     waveguide layer; -   a second active region at a second position with respect to the     waveguide layer; -   a third active region at a third position with respect to the     waveguide layer; -   a first tunnel junction below the first position and above the     second position; a second tunnel junction below the second position     and above the third position; and -   a grating at a fourth position with respect to the waveguide layer; -   wherein a first product of an overlap of the first vertical optical     mode with the grating and an overlap of the first vertical optical     mode with the first, second, and the third active regions is greater     than a second product of an overlap of the second vertical optical     mode with the grating and an overlap of the second vertical optical     mode with the first, second, and the third active regions.

Example 42. The laser of Example 41, wherein the first optical mode is a fundamental vertical optical mode supported by the waveguide layer and the at least one second vertical optical mode is a second order vertical optical mode and/or a third order vertical optical mode supported by the waveguide layer.

Example 43. A laser comprising:

-   a waveguide layer configured to supports at least a fundamental     vertical optical mode; -   a ridge layer above the waveguide layer; -   a high index layer between the waveguide layer and the ridge layer,     the high index layer having a refractive index larger than a     refractive index of the ridge layer and a refractive index of the     waveguide layer; and -   at least two active regions within the waveguide layer, the active     regions configured to provide optical gain to the fundamental     vertical optical mode.

Example 44. The laser of Example 43, further comprising a grating within the waveguide layer, the grating configured to provide feedback for the fundamental vertical optical mode.

Example 45. The laser of Example 43, wherein a first product of an overlap of the fundamental vertical optical mode with the grating and an overlap of the fundamental vertical optical mode with the two active regions is greater than a second product of an overlap of a higher order vertical optical mode with the grating and an overlap of the higher order vertical optical mode with the two active regions.

Example 46. The laser of Example 43, further comprising a tunnel junction between the two active regions.

Example 47. The laser of Example 46, wherein the tunnel junction comprises a pn semiconductor junction.

Example 48. The laser of Example 1, wherein at least one of the first and the second active regions is inside the waveguide layer.

Example 49. The laser of Example 1, wherein the grating is inside the waveguide layer.

Example 49. The laser of Example 1, wherein the tunnel junction is inside the waveguide layer.

Group 3

Example 1. A laser comprising:

-   a waveguide layer configured to support a vertical fundamental     optical mode; -   a ridge layer above the waveguide layer; -   a high index layer between the waveguide layer and the ridge layer,     the high index layer having a refractive index larger than a     refractive index of the ridge layer and a refractive index of the     waveguide layer; and -   an active region within the waveguide layer, the active region     configured to provide optical gain to the fundamental mode.

Example 2. The laser of the Example 1, further comprising a grating within the waveguide layer, the grating configured to provide feedback for the fundamental mode.

Example 3. The laser of the Example 1, wherein the waveguide layer comprises a first plurality of layers comprising a first material having a first refractive index and a second plurality of layers comprising a second material having a second refractive index.

Example 4. A laser comprising:

-   a waveguide configured to support a vertical fundamental optical     mode and at least one vertically confined higher order optical mode; -   an active region at a first position with respect to the waveguide; -   a grating at a second position with respect to the waveguide; -   wherein at least one of the first position or the second position     overlaps with a null of at least one higher order mode.

Terminology

While the foregoing detailed description discloses several embodiments, it should be understood that this disclosure is illustrative only and is not limiting. It should be appreciated that specific configurations and operations in accordance with certain embodiments described herein can differ from the particular example described herein, and that the example apparatus and methods described herein can be used in other contexts. Additionally, components can be added, removed, and/or rearranged. Additionally, processing steps can be added, removed, or reordered. A wide variety of designs and approaches are possible.

Various modifications to the embodiments described herein may be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of this disclosure. Thus, the claims are not intended to be limited to the embodiments shown herein, but are to be accorded the widest scope consistent with this disclosure, the principles and the novel features disclosed herein. The word “exemplary” is used exclusively herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. Additionally, a person having ordinary skill in the art will readily appreciate, the terms “upper” and “lower” are sometimes used for ease of describing the figures, and indicate relative positions corresponding to the orientation of the figure on a properly oriented page, and may not reflect the proper orientation of the device as implemented.

Certain features that are described in this specification in the context of separate embodiments also can be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment also can be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. Further, the drawings may schematically depict one more example processes in the form of a flow diagram. However, other operations that are not depicted can be incorporated in the example processes that are schematically illustrated. For example, one or more additional operations can be performed before, after, simultaneously, or between any of the illustrated operations. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the embodiments described herein should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products. Additionally, other embodiments are within the scope of the following claims. In some cases, the actions recited in the claims can be performed in a different order and still achieve desirable results. 

1. A laser comprising: a waveguide layer configured to support a fundamental vertical optical mode and at least a first higher order vertical optical mode; a first active region at a first position with respect to the waveguide layer; a second active region at a second position with respect to the waveguide layer; a tunnel junction between the first and the second positions; and a grating at a third position with respect to the waveguide layer; wherein the first and the second positions of the first and the second active regions, and the third position of the grating are configured to make a first lasing threshold for the fundamental vertical optical mode smaller than a second lasing threshold for the first higher order vertical optical mode.
 2. (canceled)
 3. The laser of claim 1, wherein the grating overlaps with a peak of the fundamental vertical optical mode.
 4. The laser of claim 1, wherein the grating overlaps with a null of the first higher order optical mode.
 5. The laser of claim 1, wherein the waveguide layer is configured to support a second higher order optical mode, and the grating overlaps with a null of the first or the second higher order optical mode.
 6. (canceled)
 7. The laser of claim 1, wherein a first product of an overlap of the fundamental vertical optical mode with the grating and an overlap of the fundamental vertical optical mode with the first and the second active regions is greater than a second product of an overlap of the first higher order vertical optical mode with the grating and an overlap of the first higher order vertical optical mode with the first and the second active regions.
 8. The laser of claim 5, wherein a first product of an overlap of the fundamental vertical optical mode with the grating and an overlap of the fundamental vertical optical mode with the first and the second active regions is greater than a second product of an overlap of the second higher order vertical optical mode with the grating and an overlap of the second higher order vertical optical mode with the first and the second active regions.
 9. The laser of claim 1, wherein the first lasing threshold for the fundamental vertical optical mode comprises a first threshold injection current and the second lasing threshold for the first higher order vertical optical mode comprises a second threshold injection current larger than the first threshold injection current, wherein an injection current larger than the first threshold current provided to the laser generates an optical output beam, and wherein a contribution of the first higher order vertical optical modes in the output optical beam being suppressed with respect to a contribution of the fundamental vertical optical mode by at least 10 dB.
 10. The laser of claim 5, wherein the first and the second positions of the first and the second active regions, and the third position of the grating are further configured to make the first lasing threshold for the fundamental vertical optical mode smaller than a third lasing threshold for the second higher order vertical optical mode.
 11. The laser of claim 10, wherein the first lasing threshold for the fundamental vertical optical mode comprises a first threshold injection current, the second lasing threshold for the first higher order vertical optical mode comprises a second threshold injection current, the third lasing threshold for the second higher order vertical optical mode comprises a third threshold injection current, wherein the third threshold current is larger than the second threshold injection current and the second threshold current is larger than the first threshold injection current.
 12. The laser of claim 11, wherein an injection current larger than the first threshold current is provided to the laser generates an optical output beam, and wherein contributions of the first and the second higher order vertical optical modes in the output optical beam are suppressed with respect to a contribution of the fundamental vertical optical mode by at least 10 dB.
 13. (canceled)
 14. The laser of claim 1, wherein an optical power of light outputted from the laser is between about 10 mW and about 50 W.
 15. (canceled)
 16. The laser of claim 1, wherein the waveguide layer comprises a first plurality of layers comprising a first material having a first refractive index and a second plurality of layers comprising a second material having a second refractive index.
 17. (canceled)
 18. The laser of claim 1, wherein at least one of the first and the second active regions is within the waveguide layer.
 19. The laser of claim 1, wherein the grating is within the waveguide layer.
 20. The laser of claim 1, wherein the waveguide layer is between and vertically confined by a first region and a second region, the first region comprising a first material having a first refractive index, the second region comprising a second material having a second refractive index, the waveguide layer comprising a third material having a third refractive index, the first refractive index less than the third refractive index, and the second refractive index less than the third refractive index.
 21. (canceled)
 22. (canceled)
 23. (canceled)
 24. (canceled)
 25. (canceled)
 26. (canceled)
 27. The laser of claim 1, wherein the third position is above the second position and below the first position.
 28. (canceled)
 29. The laser of claim 1, wherein the third position overlaps with the tunnel junction.
 30. A method for designing a laser comprising a waveguide layer, at least two active regions, and a grating, the method comprising: providing a first position of a first active region, a second position of a second active region, a third position of the grating; calculating at least a first vertical optical mode and at least one second vertical optical mode supported by the waveguide layer for the first and the second positions of the first and second active regions and the third position of the grating; adjusting the positions of the active regions and the grating such that a first product of an overlap of the first vertical optical mode with the grating and an overlap of the first vertical optical mode with the first and the second active regions is greater than a second product of an overlap of the at least one second vertical optical mode with the grating and an overlap of the at least one second vertical optical mode with the first and the second active regions; re-calculating at least the first vertical optical mode and the at least one second vertical optical mode and determining perturbations of at least the first vertical optical mode and the at least one second vertical optical mode resulting from the adjusted positions of the first and the second active regions and the grating; calculating a difference between the first product and the second product; adjusting, if the difference is less than a threshold value, the positions of the first and the second active regions and the grating such that the first product is larger than the second product.
 31. (canceled)
 32. (canceled)
 33. (canceled)
 34. The method of claim 30, further comprising providing a fourth position of a tunnel junction between the first and the second positions.
 35. The method of claim 34, calculating the first vertical optical mode and the second vertical optical mode comprises, calculating the first vertical optical mode and the second vertical optical mode for the fourth position of the tunnel junction.
 36. (canceled)
 37. (canceled)
 38. (canceled)
 39. (canceled)
 40. (canceled)
 41. A laser comprising: a waveguide layer configured to support a first vertical optical mode and at least a second vertical optical mode; a first active region at a first position with respect to the waveguide layer; a second active region at a second position with respect to the waveguide layer; a third active region at a third position with respect to the waveguide layer; a first tunnel junction below the first position and above the second position; a second tunnel junction below the second position and above the third position; and a grating at a fourth position with respect to the waveguide layer; wherein a first product of an overlap of the first vertical optical mode with the grating and an overlap of the first vertical optical mode with the first, second, and the third active regions is greater than a second product of an overlap of the second vertical optical mode with the grating and an overlap of the second vertical optical mode with the first, second, and the third active regions. 42-51. (canceled) 